cpu/stm32f4: optimized GPIO driver implementation
SQUASH: cpu: fixed some typos
SQUASH: cpu/f4: changed order of irq enabling seq
cpu/nrf51822: added HFCLK initialization
boards: added CLOCK defines to NRF based boards
add hdc1000 sensor driver
add test app for hdc1000 sensor
cpu/cc2538: added extern C for rfcore.h
pktqueue: fix alignment order

Alignment order of `pktqueue_node_t` does not align with `priority_queue_node_t`
Simplify rpl_clear_routing_table()

Instead of looping over the entire table and setting every entry to 0, memset the entire table at once.
boards/stm32f4discovery: fix typo in comment for section
[SQUASH ME] pktqueue: fix unittests
Basic DAC Driver, implementation on stm32f4discovery
native: add missing syscall declarations
native: coding conventions
native: add time syscalls
Merge pull request #2088 from backenklee/discovery_typo

boards/stm32f4discovery: fix typo in comment for section
Merge pull request #2005 from haukepetersen/fix_nrf_enablehfclk

cpu/nrf51822: added HFCLK initialization
Merge pull request #2087 from Lotterleben/rpl_simplify_routingtable_init

Simplify rpl_clear_routing_table()
Merge pull request #2085 from authmillenon/fix-pktqueue

pktqueue: fix alignment order