The new function (mbed_set_cpu_core_definitions()) should always be
called as it has defintions needed by Mbed OS to work with that
particular MCU core.
mbed_set_cpu_core_options() should only be called if a third-party
toolchain file has not been used as it has compile options that can
cause conflicts.
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.