net/gcoap: add table of contents to module doc

boards: pic32-wifire: Add LEDs support

boards: pic32-clicker: Add LEDs support

cpu/stm32f1: remove unused cpu_clock_scale

pkg/openthread:  core and tests

drivers/netdev: added more comprehensive doc

net/gcoap: some more minor improvements

newlib: Split newlib into one generic part and one default syscalls part

- fixed/unified indention throughout the file
- use same style for @brief blocks
- removed colons from end of brief descriptions
- fixed in and out annotations for function parameters

net/gcoap: moved implemenation status to gcoap.h

cpu/kinetis: prevent use of symlinks for ld-scripts

netdev.h: remove indirect reference to netdev2 from doc

pkg : remove aversiveplusplus

make: fix link to quickstart guide

tests: enhance xtimer_msg_receive_timeout

    - correct test period from 100ms to 1000ms, to match description
    - moved description to separate README.md
    - changed timer message type from 44 to 42, for nerdiness
    - minor code enhancements

dist: tools: coccinelle: add documentation

rpl: drop DAO-ACK when received via multicast

drivers/dsp0401: add implementation for 4 digits alphanumeric display

Doxygen:  Minor series of fixes for the documentation

kw2xrf: use static inline

ndp: warn gcc-7 about intentional fall-through

shell: fix _ccnl_content command

Since gcc-7 `Wimplicit-fallthrough` is activated by using `-Wextra`.
This leads to the following problem when compiling `gnrc_networking`:

```
RIOT/sys/net/gnrc/network_layer/ndp/internal/gnrc_ndp_internal.c: In function ‘gnrc_ndp_internal_set_state’:
RIOT/sys/net/gnrc/network_layer/ndp/internal/gnrc_ndp_internal.c:106:15: error: this statement may fall through [-Werror=implicit-fallthrough=]
   t = ipv6_iface->reach_time;
  ~~^~~~~~~~~~~~~~~~~~~~~~~~
RIOT/sys/net/gnrc/network_layer/ndp/internal/gnrc_ndp_internal.c:109:9: note: here
case GNRC_IPV6_NC_STATE_DELAY:
         ^~~~
```

The fall-through in this code is intentional. There are several ways to
warn the comiler about such intentional fall-throughs, which include
e.g. attributed empty statements (`__attribute__ ((fallthrough));`).
I don't like tis approach however. The best way would probably be to
remove this fall-through from the code. However, to keep the diff
minimal, and since ndp will change in the future, I went for warning
the compiler using comments.

The compiler checks comments for several *fall through* regexs to
decide whether a fallthrough was intentional or not.

You can read more about this gcc option in [1]. A note about
fallthrough comment regexs is at the bottom of this article.

[1] https://developers.redhat.com/blog/2017/03/10/wimplicit-fallthrough-in-gcc-7/