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
nrf51: halt works better
CI: removed AC in extern C check
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
CI: replaced head with cat and filtered out tools for extern "C" check.
Add TERMFLAGS to info-build of Makefile.buildtests

As requested in #1559
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
core/msg: use disable/restoreIRQ

* needed to change internal `msg_send` to allow external disabling of interrupts
core/msg: coding convention cleanup
spark-core fix UART speed
Merge pull request #2073 from aep/master

nrf51: halt works better
Merge pull request #2086 from mehlis/spark-core-fix-uart-freq

spark-core fix UART speed
board/spark-core: enabled C++
Merge pull request #2032 from haukepetersen/fix_sparkcore_cpp

board/spark-core: enabled C++
Merge pull request #1887 from LudwigOrtmann/msg_new_irq_api

core/msg: use disable/restoreIRQ