Allow the autoupdater to work when updating from the removed targets.

These devices have never actually existed; the corresponding support
strings have been removed from the stock firmwares as well.

It seems the device was lost during the OpenWrt -> LEDE migration.

This reverts commit fcce9e52.

The x86-generic kernel does not fit into 4MB.

This will allow upgrading using the broken sysupgrade scripts in OpenWrt
CC without losing the configuration.

Fixes #1010

Fixes #1126

The Airgateway LR is the same device as the normal Airgateway but with an detachable antenna. The Pro version is a cheap dual band device.

Fixes #1084

Hardware is very similar to Archer C5v1 and C7v2.

Tested factory install and autoupdater.

IBSS Mesh on 2.4 and 5 GHz is working fine.

The only Ethernet Port is used for config mode and afterwards handled
as WAN Port.

Completely untested for now.

Closes #835

The TL-WR940N doesn't have detachable antennas. Make v1-v3 match v4.

Switch sha512sum to sha256sum to match updated autoupdater.

The size of the factory images is limited to 4MB, which caused build
failures when many additional packages were included.

Rather than moving the device to ar71xx-tiny, we ignore the factory images
and just build the sysupgrade. This way, the whole flash is usable for
Gluon.

This means that installing Gluon on these devices will now require to flash
a plain LEDE image first, and then upgrading using the Gluon sysupgrade
image.

Fixes #1020

To fix the autoupdater.

Remove obsolete x86-xen_domu definition, it has been merged into
x86-generic.

Fixes #1022

These devices don't have unique MAC addresses with OpenWrt/LEDE at the moment
(see #993).

Closes #844

Fixes #961