boards: adjusted TIMER_0 defines for NRF based b.

adjusted for
- pca10000
- pca10005
- yunjia-nrf51822
cpu/nrf51822: fixed timers

- changed TIMER_0 to be a 24-bit timer -> see Nordic PAN #32
- cleaned up timer initialization code
- added check if an interrupt channel was active
iot-lab_M3: flashing works fine w/o device_desc
Merge pull request #1827 from haukepetersen/fix_nrf_timer

cpu/nrf51822: fixed timers
Merge pull request #1996 from OlegHahm/iot-lab_m3-no_device_desc

iot-lab_M3: flashing works fine w/o device_desc
ipv6: fix IPv6 address matching algorithm
Merge pull request #2007 from authmillenon/ipv6-match-fix

ipv6: fix IPv6 address matching algorithm
drivers/at86rf21: add and improve debug messages
Merge pull request #2008 from thomaseichinger/at86rf23x-debugs

drivers/at86rf21: add and improve debug messages
cpu/nrf51822: added HFCLK initialization
boards: added CLOCK defines to NRF based boards
cpu/nrf51822: added support for NRF51 radio