boards/frdm-kw41z: autoload fxos8700 with saul_default

tests/eepreg: remove duplicate FEATURES_REQUIRED

at86rf2xx/at86rf2xx_netdev: improve documentation.

gnrc_icmpv6_error / gnrc_ipv6: fixes for unspecified address

It just doesn't makes sense to handle them any further

Check for:

 - if it exists (critical error condition -- non-IPv6 headers should
   not trigger these functions) => assert
 - if it has a multicast source (that shouldn't really happen but
   people might try weird stuff ;-)
 - if it has an unspecified source (can't determine receiver of error
   message => don't send it, don't build it)

 gnrc_ipv6: fix _fill_ipv6_hdr() for pure IPv6 packets

If a packet only contains IPv6 and IPv6 extension header snips (e.g. if
the IPv6 packet has no payload or if an extension header was not
pre-parsed)

gnrc_icmpv6_error: fix and use where appropriate

It was not obvious where `dev->idle_state` was saved.

This is aimed to make the usage code of this module more readable.

The stack might still wants to do something with the packet after
copying it to the error message.

gnrc/pkt: Add const keyword to length functions

sys/crypto: optimize AES footprint (resurrected)

The dependency from eepreg to FEATURES_REQUIRED is already defined in
`sys/Makefile.dep` so should not need to be duplicated in the
application Makefile.

USEMODULE += crypto_aes_precalculated enables the precalculated T tables (the old
code).
USEMODULE += crypto_aes_unroll causes loops to be unrolled.

ethos: Only accept frame if previous frame is read