cpu/stm32f4: optimized GPIO driver implementation
SQUASH: cpu: fixed some typos
SQUASH: cpu/f4: changed order of irq enabling seq
cpu/nrf51822: added HFCLK initialization
boards: added CLOCK defines to NRF based boards
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
add hdc1000 sensor driver
add test app for hdc1000 sensor
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.
cpu/cc2538: added extern C for rfcore.h
Add TERMFLAGS to info-build of Makefile.buildtests

As requested in #1559
pktqueue: fix alignment order

Alignment order of `pktqueue_node_t` does not align with `priority_queue_node_t`
Merge pull request #2036 from haukepetersen/fix_openmote_cpp

boards: enable C++ for CC2538 based boards