Better granualarity and erro checking of the board's MCG settings

  Allow for complete MCG_C2 definition from the boart.h file
  Moved #ifdef out of code by setting default values for
  Allow for individule bit setting in MCG_C2 for
    BOARD_EXTCLOCK_MCG_C2
    BOARD_MCG_C2_FCFTRIM
    BOARD_MCG_C2_LOCRE0
  Added range and sanity checking