cpu/samd21: increase default stack size

Provide enough space to run `examples/default`.
examples/rpl_udp: blacklist samr21-xpro

It does not have enough RAM to run this application at the moment.
rpl: cleanup, changing structdefs into typedefs
Merge pull request #2234 from cgundogan/rpl_struct_typedef_cleanup

rpl: cleanup, changing structdefs into typedefs
Merge pull request #2229 from LudwigOrtmann/issue-2228

cpu/samd21: increase default stack size
netdev: Introduce setter for event handler thread
[SQUASH ME] netdev: adapt tests
[SQUASH ME] tests: check if pid in receive callback is correct
[SQUASH ME] netdev: move from set_event_handler() method to member variable
[SQUASH ME] netdev: remove some now unnecessary changes
[SQUASH ME] nativenet: fix broken initializer
drivers/cc110x: Allow packet payload up to 255 bytes
drivers/cc110x: Adapt rx/tx to changed rcv event handling