sys/auto_init: improve documentation

makefiles/cflags.inc.mk: add -Wmissing-include-dirs flag 

fmt: Add fmt_strnlen function

gnrc_sixlowpan: fix order of gnrc_sixlowpan_msg_frag_t

gnrc_sixlowpan_frag: expose (parts of) reassembly buffer

tests: add some benchmarks

embunit: Const test case names

gcoap: don't allocate memo for clients without response handlers

makefile.iotlab.single: add IoT-LAB testbed support for one node

Add a target to show the value of IOTLAB_NODE. Can be used to get the result of
using IOTLAB_NODE=auto or IOTLAB_NODE=auto-ssh.

Format the output to be '0' on success and check it with grep.

This makes command fail when they failed on IoT-LAB.

Usage:

    make BOARD=iotlab-m3 IOTLAB_NODE=auto-ssh all flash term

Add support to select node number in auto mode

Add support to do flash/reset/term on an IoT-LAB node.
It also allow running test using 'testrunner'.

Configuration variables are:

* `IOTLAB_NODE` which should be set to your node url
  * The full url including site to use from your computer `m3-1.grenoble.iot-lab.info`
  * The short url when used on the IoT-LAB frontend `m3-1`
* `IOTLAB_EXP_ID` for your experiment id for flash and reset.
  By default it tries to use your currently running experiment if you have only one
* `IOTLAB_USER`: is read from `${HOME}/.iotlabrc` as saved by `iotlab-auth`
  * It is expected to have run `iotlab-auth` beforehand.

pkg: add support for the tinycrypt library

core: add more stacksize defines

gnrc/pktbuf: Const correctness on gnrc_pktbuf_add

While working on #9352 I noticed that the order of members in the
`gnrc_sixlowpan_msg_frag_t` struct costs us 4 bytes in RAM due to byte
alignment. This PR fixes the order of members, so they are the most
packed.

This exposes the parts of the reassembly buffer to be usable as context
as proposed in #8511.

I only exposed *parts of* for two reasons:

1. I don't need to expose further types (like `rbuf_int_t`), that are
   not of interest outside of fragmentation.
2. This allows for an easy future extension for the virtual reassembly
   buffer as proposed in [[1]].

This makes this change a little bit more involved, because instead of
just renaming the type, I also need to add the usage of the `super`
member, but I think in the end this little preparation work will be
beneficial in the future.

[1]: https://tools.ietf.org/html/draft-watteyne-6lo-minimal-fragment-01#section-3

test/sizeof_tcp: fixed typo in Makefile

net/gcoap: Refine Observe re-registration

makefiles/vars.inc.mk: fixup indentation

core/mbox: doc typo fixes