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
Initial import of i2c lowlevel driver for stm32f4
boards/samr21-xpro: declare implemented features
Merge pull request #1859 from PeterKietzmann/add_stm32f4_i2c

cpu/stm32f4: added i2c driver implementation
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/lpc1768: heavy clean-up of existing code

- moved to new port structure
- rewrote startup code
- added implementation dummies for UART and timer
- switched to atmel linkerscript
- cleaned up Makefiles
board/mbed_lpc1768: heavy clean-up

- removed legacy code
- added default defines
- created periph_conf.h
Initial import of the updated srf02 driver with general i2c interface
Merge pull request #2045 from haukepetersen/fix_board_mbed

board/mbed_lpc1768: remodeled to our board/cpu structure
Merge pull request #1871 from PeterKietzmann/add_srf02_general_i2c_if

drivers/srf02: adapted ultrasonic ranger driver to general i2c interface
cpu/cc2538: made _kill(_r) syscalls weak