rpl: minor cleanup

gnrc: add netopt_t to string map

core: missing restoreIRQ in mutex_unlock

`mutex_unlock()` did not restore IRQsbefore yielding for the woken up
thread.

fib: allow for multiple instances of FIB

rpl: omit dodag id in DAO(-ACK) for global instances

universal_address: minor dox fixes

gnrc_sixlowpan_iphc: set correct length for IPv6 header

cpu/stm32f0: RTC implementation

tests: Tests for RTC clock

cortexm_common: don't compare with r13/sp in assembly, fix #3664

gnrc_ipv6.c: remove padding added by lower layers

iot-lab_M3: rename to iotlab-m3

c++: Define some support functions required by GCC

gnrc_sixlowpan: compare with actual packet size for fragmentation

Not with the uncompressed version

cpu/cortexm_common: additional information on hardfault

wakaama: fixed errors for bootstrap mode

gnrc: make all gnrc modules sub-modules of gnrc

ipv6_addr: adapt doc to proposed changes in #3608

rpl: make urls in doc more verbose

ng_rpl: auto address configuration

gnrc_rpl: fix dependencies

ndp: move type definitions out of ng_ context