add RTOS support for Nucleo STM32F302R8

This works with the rtos_basic project.

Initial SP from
mbed/libraries/mbed/targets/cmsis/TARGET_STM/TARGET_NUCLEO_F302R8/TOOLCHAIN_ARM_MICRO/startup_stm32f302x8.s

Please review the other values (task count, stack size, clock).

Thanks,
Lawrence