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/cc2538: made _kill(_r) syscalls weak
board/openmote: enabled C++ support
board/cc2538dk: enabled C++ support
core/cpu/boards: removed fw_puts from RIOT
tools/licenses: add 3c-BSD-atmel3 pattern
tools/licenses: add 3c-BSD-nordic pattern
license: added missing license for msp430 time.h
tools/licenses: remove unused 3c-BSD-clark
cpu/board: update stale redbee license headers
lint: tests: fix improper use of format specifiers
tools/licenses: add 1c-BSD-stanford pattern