To simplify board definitions and for unification between samd2x and
newer models, don't use the GCLK bitmask in board definitions.
Instead use the GCLK index and generate the bitmask when needed.
With the introduction of BOARDSDIR external boards can re-use common
code of BOARDS present in RIOTBASE. To be able to do this file
references may not use $(BOARD) since BOARD will be set by the
external BOARD.