samr21-xpro: initial import for the samr21-xpro board
Merge pull request #1701 from thomaseichinger/samr21

cpu/board: Initial import of samr21-xpro
drivers: import PIR motion sensor driver
SQUASHME: remove missing GPIO workaround

Addresses https://github.com/RIOT-OS/RIOT/pull/1714#discussion_r18046571
SQUASHME: make private API static

Addresses https://github.com/RIOT-OS/RIOT/pull/1714#discussion_r18046365
SQUASHME: blacklist boards, provide defaults WIP
SQUASHME: add samr21-xpro define