Update UART pin names & add MBED_CONF_TARGET_STDIO_UART overrides

Add standard pin-names validation script & tests

Add bare metal support to EFM32 targets ...

Modify scatter files to add heap load region, and remove hard coded
values. Add supported c libs and bare metal profile to `targets.json`
configurations. Affects the following targets: EFM32GG_STK3700,
TB_SENSE_12, and EFM32GG11_STK3701.

CMake: Refactor Silicon Laboratories targets ...

Refactor all Silicon Laboratories targets to be CMake buildsystem targets. This removes
the need for checking MBED_TARGET_LABELS repeatedly and allows us to be
more flexible in the way we include MBED_TARGET source in the build.

A side effect of this is it will allow us to support custom targets
without breaking the build for 'standard' targets, as we use CMake's
standard mechanism for adding build rules to the build system, rather
than implementing our own layer of logic to exclude files not needed for
the target being built. Using this approach, if an MBED_TARGET is not
linked to using `target_link_libraries` its source files will not be
added to the build. This means custom target source can be added to the
user's application CMakeLists.txt without polluting the build system
when trying to compile for a standard MBED_TARGET.

CMake: Add support for SiLabs EFR32MG12 targets

stack: armc: Workaround config passing bug ...

Workaround a bug where the boot stack size configuration option is not
passed on to armlink, the Arm Compiler's linker. Prefer
MBED_CONF_TARGET_BOOT_STACK_SIZE if present, as this is what the
configuration system should provide. Fall back to MBED_BOOT_STACK_SIZE
if MBED_CONF_TARGET_BOOT_STACK_SIZE is not defined, as in the case of
buggy tools. If both MBED_CONF_TARGET_BOOT_STACK_SIZE and
MBED_BOOT_STACK_SIZE are not defined, then we fall back to a hard-coded
value provided by the linkerscript. See
https://github.com/ARMmbed/mbed-os/issues/13474 for more information.

Use boot stack size from config system ...

To allow overriding of the boot stack size from the Mbed configuration
system, consistently use MBED_CONF_TARGET_BOOT_STACK_SIZE rather than
MBED_BOOT_STACK_SIZE.

Fixes #10319

EFM32: move __Vectors from header to source

EFM32: update emlib for EFR32MG12

Clean up ARM toolchain heap+stack setup in targets ...

ARM Compiler 6.13 testing revealed linker errors pointing out
conflicting use of `__user_setup_stackheap` and
`__user_initial_stackheap` in some targets. Remove the unwanted
`__user_initial_stackheap` from the targets - the setup is
centralised in the common platform code.

Looking into this, a number of other issues were highlighted

* Almost all targets had `__initial_sp` hardcoded in assembler,
  rather than getting it from the scatter file. This was behind
  issue #11313. Fix this generally.
* A few targets' `__initial_sp` values did not match the scatter
  file layout, in some cases meaning they were overlapping heap
  space. They now all use the area reserved in the scatter file.
  If any problems are seen, then there is an error in the
  scatter file.
* A number of targets were reserving unneeded space for heap and
  stack in their startup assembler, on top of the space reserved in
  the scatter file, so wasting a few K. A couple were using that
  space for the stack, rather than the space in the scatter file.

To clarify expected behaviour:

* Each scatter file contains empty regions `ARM_LIB_HEAP` and
  `ARM_LIB_STACK` to reserve space. `ARM_LIB_STACK` is sized
  by the macro `MBED_BOOT_STACK_SIZE`, which is set by the tools.
  `ARM_LIB_HEAP` is generally the space left over after static
  RAM and stack.
* The address of the end of `ARM_LIB_STACK` is written into the
  vector table and on reset the CPU sets MSP to that address.
* The common platform code in Mbed OS provides `__user_setup_stackheap`
  for the ARM library. The ARM library calls this during startup, and
  it calls `__mbed_user_setup_stackheap`.
* The default weak definition of `__mbed_user_setup_stackheap` does not
  modify SP, so we remain on the boot stack, and the heap is set to
  the region described by `ARM_LIB_HEAP`. If `ARM_LIB_HEAP` doesn't
  exist, then the heap is the space from the end of the used data in
  `RW_IRAM1` to the start of `ARM_LIB_STACK`.
* Targets can override `__mbed_user_setup_stackheap` if they want.
  Currently only Renesas (ARMv7-A class) devices do.
* If microlib is in use, then it doesn't call `__user_setup_stackheap`.
  Instead it just finds and uses `ARM_LIB_STACK` and `ARM_LIB_HEAP`
  itself.

TARGET_Silicon_Labs Setup heap limit and size

[Silicon_Labs] Support boot stack size configuration option

SiLabs: Fix alignment of execute region to 8-byte boundary ...

--legacyalign, --no_legacyalign are deprecated from ARMC6 compiler, in order to
remove deprecated flags all linker files (GCC and IAR as well to have uniformity)
should strictly align to 8-byte boundary

Changes in PR #7774 of PinNames.h should be reverted. This commit reverts those files excpet for K82F and K64F which are left as an example

Moving SD, SPIF and FLASHIAP into mbedos and refactoring features storage directory structure.

Place "MBED_WEAK" for IAR-Toolchain before the type.

Added missing include.

Replace __attribute__((weak)) with MBED_WEAK

EFM32 Make PeripheralPins.c configuration tables weakly defined to be overridable. ...

See issue "https://github.com/ARMmbed/mbed-os/issues/7424#issuecomment-404233377"

Update pinout of TB_SENSE_12 to production revision ...

* Updated pinout of EXP header from pre-production to production
* Move assignment of PF7 to SPI_2 to be compatible with the new SPI flash routing

Set up the ISR stack pointer in the right location on ARMCC ...

EFR32MG1, EFR32MG12 and EFM32PG12 didn't have a fixed ARMCC linker script yet.

EFM32: make peripherals conditional ...

* MCUs within a family like EFM32GG can omit some peripherals, e.g. EFM32GG230 doesn't have UART
* This commit adds a check to make them compilable, relevant mainly for custom boards

Allow overriding of DCDC settings per target ...

Allow custom targets to override the DCDC settings by defining EMU_DCDCINIT_STK_DEFAULT to target-specific initialization values.

Upcase all assembler file extensions

Merge pull request #4475 from c1728p9/workshop_rebase_4043 ...

Silicon Labs: Add bootloader support

[Silicon Labs] Bugfixes to using bootloader with Series 1 devices.

[Silicon Labs] Add bootloader support ...

* Make memory sections configurable in linker files
* Dynamically determine vector location in flash for NVIC relocation
* Advertise bootloader support in targets.json

Added mapping to BTN-labelled switches

[Silicon Labs] Update TB Sense 2 ...

* Update i2c driver to tackle multiple I2C object instances on different pins
* Update pinmap to enable the extra peripherals of EFx32xG12 over EFx32xG1

[Silicon Labs] Rename targets ...

Due to limitation in the mbed website backend (board names need to be <= 19 characters), we are shortening the CLI target names from THUNDERBOARD to TB.
@screamerbg