Minimal changes to get a clean compilation of the SAMD20 Xplained configuration. Still a lot of missing logic

Add configuration for the SAMD20 Xplained Pro board.  Initial check a low-effor, unverified clone of thoe SAM4L Xplained Pro board configuration

Many changes to reduce complaints from CppCheck.  Several latent bugs fixes, but probably some new typos introduced

Add a mouse interface that is similar to the touchscreen interface except that it can handle multple buttons and continuously reports positional data so that it can control a cursor

The USB HID mouse driver is now functional.  There are some usage issues as described in the top-level TODO list

STM32F4 Discovery:  Rename most everything in the board source directory to conform with the ever evolving naming standard