cpu/stm32f4: optimized GPIO driver implementation
SQUASH: cpu: fixed some typos
SQUASH: cpu/f4: changed order of irq enabling seq
net: move ping to sys/net

* moved the former ping module from `sys` to `sys/net/link_layer` and renamed to l2_ping
* use defaulttransceiver instead of cc110x
* some refactoring
net: l2_ping uses radio_packet_t instead of packet_info_t
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: moved reboot_arch for cortex cpus
cpu/cortex-mX_common: fixed reboot_arch return val
cpu/cortex-mX_common: added delay to reboot_arch
cpu/cc2538: made _kill(_r) syscalls weak
board/openmote: enabled C++ support
board/cc2538dk: enabled C++ support
Merge pull request #2047 from haukepetersen/fix_cortex_reboot

cpu: fixed and moved reboot_arch for cortex cpus
core/cpu/boards: removed fw_puts from RIOT
tests: define random default GPIOs to pass Travis builds
Merge pull request #2064 from thomaseichinger/sensor-features

tests: define random default GPIOs to pass Travis builds
tests: fix tests/posix_semaphore