platform/mulle: Add support for rev 2.x silicon Mulles.

All Mulle boards manufactured after the first batch are using
revision 2 of the K60 silicon. This requires some changes to the
software.

A new makefile define has been added in order to allow quicker
switching between the two revisions.
Build using revision 2 code:
$ make MULLE_BOARD_SERIAL_NUMBER=300
Build using revision 1 code:
$ make MULLE_BOARD_SERIAL_NUMBER=200

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>