Exception are made for emb6, nordic_softdevice_ble, openthread and semtech-loramac because their documentation contains warning/issues

The detailed documentation of some functions need the brief description
included to make sense.

Our doc builder at doc.riot-os.org was updated and now the documentation
looks weird (see http://doc.riot-os.org/group__core__msg.html#gad1353dec9af776d4caf5f4e00cec112e
e.g.). This fixes that.

Provides a script to generate a doxygen page from the
`release-notes.txt` and includes it to the doxygen build.

Fixes #6953

With some minor hand-edits I used the following chain of commands:

```sh
git rm sys/include/net/gnrc/netdev.h
git grep --name-only -i netdev2 | \
        xargs sed -i -e 's/^\(NETDEV\)2\(.*\)\( [("]\)/\1\2 \3/g' \
                     -e 's/\(netdev\)2\(.*\)\( \/\*\*<\)/\1\2 \3/I' \
                     -e 's/\(netdev\)2/\1/gI'
git add -p
git commit --amend
git ls-tree --full-tree -r HEAD --name-only | \
        grep "netdev2" | xargs -I'{}' dirname '{}' | uniq | \
        grep "netdev2" | while read dir; do
                new_dir="$(echo "$dir" | sed "s/netdev2/netdev/g")"
                git mv -f "$dir" "$new_dir"
        done
git commit --amend
git ls-tree --full-tree -r HEAD --name-only | \
        grep "netdev2" | while read file; do
                new_file="$(echo "$file" | sed "s/netdev2/netdev/g")"
                git mv -f "$file" "$new_file"
        done
git commit --amend
git grep --name-only "\<drivers_netdev_netdev\>" | \
        xargs sed -i "s/\<drivers_netdev_netdev\>/drivers_netdev_api/g"
git add -p
git commit --amend
```

- de-blacklisted nrf24l01p_settings.h from doxygen excludes
- fixed doc for that file

    - add docu for structs, defines and functions
    - exclude drivers/kw2xrf/include/overwrites.h (vendor header)
    - add missing header guards

Doxygen 1.8.12 leaves these files behind after finishing the build.

fixes tons of false warnings on cpu_conf.h headers in cpu

/tmp/RIOT/cpu/atmega1281/include/cpu_conf.h:12: warning: the name `/tmp/RIOT/cpu/atmega1281/include/cpu_conf.h' supplied as the second argument in the \file statement matches the following input files:
   /tmp/RIOT/cpu/atmega1281/include/cpu_conf.h
Please use a more specific name by including a (larger) part of the path!