Merge pull request #7222 from miri64/gnrc_ipv6_nib/feat/pl-component

sys/include/net/gnrc/ipv6/nib.h

sys/include/net/gnrc/ipv6/nib/pl.h

sys/net/gnrc/network_layer/ipv6/nib/_nib-internal.c

sys/net/gnrc/network_layer/ipv6/nib/_nib-internal.h

sys/net/gnrc/network_layer/ipv6/nib/nib_pl.c

sys/shell/commands/sc_gnrc_ipv6_nib.c

...ttests/tests-gnrc_ipv6_nib/tests-gnrc_ipv6_nib-internal.c

tests/unittests/tests-gnrc_ipv6_nib/tests-gnrc_ipv6_nib-pl.c

tests/unittests/tests-gnrc_ipv6_nib/tests-gnrc_ipv6_nib.c

tests/unittests/tests-gnrc_ipv6_nib/tests-gnrc_ipv6_nib.h