Building binaries with MicroLib and Arm compiler 6 has been failing
because of a bug in Arm compiler 6. The compiler introduces a
symbol, `__scanf_mbtowc`, which is not present in MicroLib C library.
The present commit adds a weak reference to the symbol to allow a
successful linkage. This is a temporary fix until the bug in the
compiler is fixed. It should be removed after the compiler bug is fixed.
Instead of user defined symbols in assembly files or C files,
use linker scripts to add heap and stack - this is inconsistent
with ARM std linker scripts
Refactor the Mbed 5 boot process to make is simpler and more modular.
This is done by breaking the boot sequence into 4 distinct steps -
Target setup, Toolchain setup, RTOS setup and Mbed setup. This patch
also move toolchain specific code into a per toolchain folder to make
it more maintainable.