make: made consistent use of spaces in Makefile.dep

rpl: pass rpl_instance_t directly instead of its id

sixlowpan: Broadcast message when neighbour not found in cache.

rpl: use global ip addresses instead of link-local ones

at86rf231: at86rf231_set_address_long bugfix

Core: doc: add note advising against creation of threads from within ISR

boards: added support for ST nucleo-f091

dist/tools/whitespacecheck: Do not redirect stdout.

The old code was never working, all of the middle address bytes were set
to 0x00 because of the bitwise AND being executed after the right shift.

Show new whitespace errors in Travis log for easier code review.

ng_netif: cleanup array after remove

shell: port ifconfig for ng_netif

boards: fix flashing of arduino-due and spark-core

ng_net: changed semantic of get and set command slightly

Though they are not needed in the code of `ng_netif` both `ng_pktbuf`
and `ng_netapi` are needed to use `ng_netif` in a sensible way.

cpu/stm32f0: included core_panic module

cpu/stm32f0: fixed gpio interrupt routines

net: initial import of an IPv6 neighbor cache

ng_netconf: add source length identifier

- guarded calls in case no interrupt is defined
- eneabled detection of multiple, simoultaneous events

- Fixes #2136
- Adds warning about creating threads from within an ISR.

make: allow buildtest targets to function without BUILDTEST_MCU_GROUP