Adapt start page to improve developement procedures
boards/samr21-xpro: declare implemented features
cpu/cc2538: made _kill(_r) syscalls weak
board/openmote: enabled C++ support
board/cc2538dk: enabled C++ support
Merge pull request #2050 from PeterKietzmann/adapt_README_start_page

README start page: centralize developement procedures specs, point to wiki instead of duplicating in README
Merge pull request #2036 from haukepetersen/fix_openmote_cpp

boards: enable C++ for CC2538 based boards
Merge pull request #2053 from thomaseichinger/samr21-features

boards/samr21-xpro: declare implemented features