sys/checksum: initial import including CRC16-CCITT

ndp/internal: fix pio flags for rtr adv

sys: remove hash_string module

gnrc_ipv6/whitelist: switch assignment operation to `memcpy()`

hash_string implements djb2 which is already part of `sys/hashes/`

iotlab makefiles: minor cleanups

unittests: fix for python3

Adds a checksum module which includes an implementation "the CCITT"
variant of the CRC16 algorithm.

cpu/cc2538: vendor header cleanup

FIB: remove superfluous things

In order to avoid potential naming clashes.

examples/gnrc_networking: add explanation for RIOT<->RIOT communication

microcoap_server: add instructions for FF Copper

boards: removed F_CPU defines

unittests/crypto: changed to really compare digest vs string of digest

- added missing documentation
- repaired doxygen groups

* added additional test with longer input
* uncrustified file

cpu/stm32f1: udpated vendor headers

sc_random: use a default seed if no xtimer is present

gnrc_ipv6_netif: don't advertise routers without GUAs

pkg: unify git paradigms for packages