Replace occurences of ar71xx with ath79. Remove references of ar71xx
targets which are not relevant anymore.

Closes #1993

The docs previously only listed ath9k and ath10k based chipsets as fully
supported by Gluon, however we also support mt76.

For simplicity, we don't use different MTUs for compat 14 and 15 anymore,
there's no harm in using 1532 for batman-adv-legacy as well.

[Matthias Schiffer: changed the text a bit]

Signed-off-by: Sven Eckelmann <sven.eckelmann@open-mesh.com>

As the Gluon kernel won't be compatible with the OpenWrt upstream anymore,
we don't need the vermagic hack anymore.

The file targets/$GLUON_TARGET/config becomes optional, as many targets
only used it to set the board and subtarget.

Also fix targets without subtarget.

With a backported patch from the OpenWrt trunk, it is now easy to generate an
equivalent configuration using CONFIG_ALL_KMODS.

The build will take a bit longer because all kernel module packages are actually
built even when they are not included in the image, but adding new targets
becomes a lot easier.

Also, related documentation updates and fixes.

More information on how to get correct image file naming.