make -f test-checks.mk test-ensure_value should succeed, while
make -f test-checks.mk test-ensure_value-negative should fail.

A call to `$(ensure_value x,y)` will fail with message y if x is empty, and
otherwise return x. This can be useto write more compact makefiles, while still
producing friendly error messages.

dist/tools: add build system sanity check script

tests/puf_sram: add input args to automation script

boards/frdm-kw41z: update i2c configuration

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.

Add a script to execute sanity checks on build system files.
It should prevent bad patterns to re-appear after being cleaned.

Currently adds a check for using the content of `FEATURES` instead of
`USEMODULE`.

Modules should not check the content of FEATURES_PROVIDED/_REQUIRED/OPTIONAL
Handling specific behaviors/dependencies should by checking the content of:
 * `USEMODULE`
 * maybe `FEATURES_USED` if it is not a module (== not a periph_)

gnrc/pkt: Add const keyword to length functions

sys/crypto: optimize AES footprint (resurrected)