Prefer using 'uint8_t' buffers as it is too easy to run into C sign
extension problems with 'char' buffers. Prefer using 'uint_fast8_t'
for buffer lengths as gcc does a better job compiling them on 32bit
mcus.
Signed-off-by: Kevin O'Connor <kevin@koconnor.net>
Extract out common code from avr/serial.c, sam3x8e/serial.c, and
stm32f1/serial.c into a new generic/serial_irq.c file.
Signed-off-by: Kevin O'Connor <kevin@koconnor.net>