Reworked the serial_format() function for STM32F0x
devices to take the format in the form:
data_bits - parity - stop_bits
E.g. 8 - N - 1
where data_bits exclude the parity bit.
Added a case for 7 bits data as at least the chips
STM32F0x1/STM32F0x2/STM32F0x8 support 7 bits data.
Consolidated serial_format() and uart_init()
functions into a general TARGET_STM serial_api.c
file since the functions are common to all STM targets.
Fixes#4189
Francisco J. Manno