mbed_sdk_init() is provided by mcu-driver-bootstrap, a CMake target that
has been deprecated and will be removed.

As it seems, there is a valid use case for a software hook that allows
user code execution before entering the main() function. Some of the
static objects created by pigweed modules when running tests rely on the
CMSIS-rtos being initialized beforehand.

Replace mbed_sdk_init() with explicit function definitions, specific
to the C library used in the project.

Signed-off-by: Filip Jagodzinski <filip.jagodzinski@arm.com>