core: improved doxygen documentation

improved files complient to issue #950
- thread.h
- flags.h
- debug.h
- crash.h
github: added CONTRIBUTING.md
README.md: make link a markdown link
Merge pull request #1064 from mehlis/CONTRIBUTING

github: added CONTRIBUTING.md
board: Initial import of the UDOO board

- fixed comment style and bracketing
- disabled newlib nano, added mac support
- fixed OSX compatibility in Makefile.include
Merge pull request #958 from haukepetersen/doxygen_fix_core_1

core: improved doxygen documentation
examples/tests: blacklisted udoo board

blacklisted for:
- ccn-lite-client
- ccn-lite-relay
- rpl_udp
- test_net_if
- test_pnet
Merge pull request #1108 from haukepetersen/board_udoo

board: Initial import of the UDOO board