The file always exist so no need to do '-include'.

Replaced using:

    sed -i 's|-\(include $(RIOTCPU)/.*/Makefile.features\)|\1|' \
        $(git grep -l  '$(RIOTCPU)/.*/Makefile.features' boards)

    Adapt the periph ADC implementation to use vendor defines where
    ever possible. Remove duplicate or obsolete defines and adapt
    board configuration as required.

    Rework SPI periph driver to use proper RIOT GPIO API functions.
    Also cleanup header files by using vendor defines and remove
    obsolete code. Further, adapt board config accordingly.

    The SPI bus frequency/clock is calculated relative to the MCUs
    core clock. Currently all boards use the default 32MHz, hence
    prescaler settings for SPI are all the same. This PR moves the
    default config for 32MHz to the CPU and allows to be overriden
    by board config if needed.

- replace brief with name where required
- remove some unwanted empty lines
- fix missing closing doxygen name block

* Remove '-Oihex' and '-Obinary' from OFLAGS for all boards
  It is now provided by the Makefile.include rule.

Changed the configuration of the OpenMote board to fit the new
cc2538 format defined in the previous commit. That is, from a defines
based format to a struct based format.

Not link-able, since NDP and NC are missing (intentionally)

adapted boards are: cc2538dk, openmote-cc2538, remote-pa,
remote-reva, and remote-revb.

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
```

- adapted the SPI driver
- adapted all boards using the CPU

Enable directly from Makefile target 'flash' using cc2538-bsl script,
and make this the default flashing method for this platform.