* can be enabled by defining Makefile macro SERIAL_MOUSE_MICROSOFT_ENABLE or
SERIAL_MOUSE_MOUSESYSTEMS_ENABLE.
* Serial implementation can be chosen via SERIAL_MOUSE_USE_SOFT and
SERIAL_MOUSE_USE_UART macros
* UART configuration still has to be done in config.h: I added working clauses
for both mouse protocols to ps2_usb's config.h
