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.

MicroLib is the lightweight C lib for the Arm toolchain and
used as the default C lib on bare metal builds with the Arm
toolchain. It requires two separate memory regions for stack
and heap.

The change is based on nRF52840.sct.

Add license identifier to files which Arm owns the copyright to,
and contain either BSD-3 or Apache-2.0 licenses. This is to address
license errors raised by scancode analysis.

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.

The same default value is provided in the GCC_ARM linker file.

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

nRF52 I2CSlave Implementation

NRF5x targets: Replace UARTSerial references with BufferedSerial

`BufferedSerial` is `UARTSerial` renamed to convey the original purpose
of the class. It is the recommended buffered I/O serial class.

All targets must implement soft_- and hard_power_on/off() functions which are practically same what onboard_modem_api offered.
These were seen as a duplicate features and therefore we removed this.
All targets involved have been updated to reflect the changes

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.

Nordic NRF52832 cache ON