cpu/nrf51822: added HFCLK initialization
boards: added CLOCK defines to NRF based boards
add hdc1000 sensor driver
add test app for hdc1000 sensor
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
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
Merge pull request #2070 from jfischer-phytec-iot/wip@hdc1000

Sensor Driver for HDC1000