cpu/stm32f4: optimized GPIO driver implementation
SQUASH: cpu: fixed some typos
SQUASH: cpu/f4: changed order of irq enabling seq
Basic DAC Driver, implementation on stm32f4discovery
Initial import of nrf24l01p lowlevel functions
Merge pull request #1704 from PeterKietzmann/add_nrf24l01p_lowlevel

drivers/nrf24l01+ : added low-level functions for nrf24l01+ transceiver
cpu/stm32f3: added low-level PWM driver
board/stm32f3discovery: added PWM

- added PWM feature
- added PWM definitions to periph_conf.h
Merge pull request #2093 from haukepetersen/add_stm32f3_pwm

cpu/stm32f3: added PWM driver
Merge pull request #2018 from brummer-simon/devel-dac_driver

Basic DAC Driver, implementation on stm32f4discovery
Merge pull request #1868 from haukepetersen/fix_stm32f4_gpiootp

cpu/stm32f4: optimized GPIO driver implementation