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
nrf51822: test arguments for read/write even in stubs
nrf51822: Fix indentation in lpm_arch.c
nrf51822: error was not checked b/c of wrong signedness
nrf51822: Suppress unused argument warnings
cpu: added `extern "C"` to headers
Merge pull request #2078 from Kijewski/hello-world-warnings

nrf51822: hello-world builds without warnings
Merge pull request #1803 from BytesGalore/extern_C_in_headers_cpus

cpu: added `extern "C"` to headers
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.