Fix NRF51822 PWM and Serial:
- imlement pwmout_free()
- check UART interrupt type carefully
- implement serial_set_flow_control()
- add serial_clear() stub
Merge pull request #534 from anpilog/master

HAL: nrf51822 - Few fixes for PWM and Serial
[NUCLEO_F401RE] enhance gcc ld file
Merge pull request #543 from dbestm/master

CMSIS: NUCLEO_F401RE - ARM GCC ld RAM size correction