stmhal - add led support. Add netduino_plus_2 support

Tweaked a couple of the USB files to compile if neither dev nor host was defined.
Tested on netduiono plus 2 and stm32f4discovery boards
stmhal - add pin mapping, gpio, exti, usrsw
Merge pull request #346 from dhylands/stmhal-led

stmhal - add led support. Add netduino_plus_2 support