- Split mbed-core and mbed-rtos into -sources and -flags targets ...

- Fix some logic errors in top level cmakelists related to building greentea tests
- Fix mbed_create_distro() for OBJECT libraries

Update GCC except.S to support ARMC6 ...

ARM Compiler 6 supports GNU-style assembly with armclang. Instead of
relying on armasm with the legacy syntax, GCC except.S is updated so
that it can be used with ARM as well as GCC_ARM toolchains. CMake is
updated to use this version.

CMake: remove IAR references as they are not supported

CMake: add mbed-os and mbed-baremetal targets ...

mbed-os consists of mbed-core and mbed-rtos
mbed-baremetal consists of mbed-core

The main change is for mbed-core. Changing from object library to be interface. This way it allows us to do the above to have 2 main targets for users to use.

This should be backward compatible change as mbed-os target we used contains the same files/options as previously set.

CMake: Rename CMake targets ...

* mbed-os renamed mbed-core
* mbed-os-<COMPONENT> renamed mbed-<COMPONENT>

CMake: Add support for platform dir

Fix IAR assembler errors on except.S ...

Fixes two errors produced on except.S by the IAR assembler (as seen in
issue #13200).

The first: "'ALIGN' is illegal. Use directive ALIGNROM or ALIGNRAM (the
first parameter is raised to the power of 2, the second is a 32-bit fill
value)."
Fixed by changing ALIGN to ALIGNROM 2 to align to a 4-byte boundary
(mirroring what ALIGN does in the ARM except.S).

The second (on THUMB): "Must be placed inside a section"
Fixed by moving THUMB after the SECTION directive so that it affects
that section.

Streamline crash data access ...

Use correctly-typed external definition for the crash data region, and
eliminate unnecessary pointer indirection.

Results in a small ROM saving even with crash capture disabled, as there
was a pointer for the fault context store in either case. The pointer
isn't needed, as the context store location is fixed according to the
configuration flag.

Optimise fault handler assembly ...

Fault handler assembly was very simplistic. Optimise for a 112 byte saving.

Reformat except.S ...

Tidy up formatting, and make all 3 toolchain versions more consistent,
for ease of diffing and transferring changes between them.

platform: fix SPDX identifiers

Fixed baremetal heap and stack initialisation. ...

Enabled heap_and_stack test for baremetal.
Added a test to check that global variables are initialised.

In mbed_sdk_boot:
- Added initialisation for mbed_stack_isr_start/size and mbed_heap_start/size for all toolchains.
- ARM toolchain:
   - Added call to mbed_toolchain_init() to initialise global variables.
   - Moved microlib initialisation code from mbed_retarget.cpp to mbed_sdk_boot.c.
- IAR toolchain: there is no equivalent to a software init hook that can be called. __low_level_init() was used but since this function is called before RAM initialisation, it cannot be used to initialize global variables. Defined a new __mbed_init() function called from IAR startup file instead.

Avoid crashes during fault handler ...

If the fault handler was hit before the stdio console was used and
initialised, the initialisation code caused a "mutex in ISR" trap,
stopping the register dump from happening.

Temporarily set the `error_in_progress` flag at the top of the fault
handler, and restore it before calling `mbed_error`. Take the
opportunity to suppress fault dumps on recursive crashes, much as is
done inside `mbed_error`.

The candidate code to enable correct crash report for HW fault crashes.

mbed fault handler: changed the 2nd parameter type.

Fixes the bug of having &mbed_fault_context argument instead of mbed_fault_context in mbed_fault_handler.

Move TARGET directories to source