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