These devices are identical to the TL-WR941 v4 & v5, so we don't need to
test them explicitly.

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

Use the same image for sysupgrade on WZR-HP-AG300H and WZR-600DHP, but use
different factory images.

This reverts commit 8ad3c881.

We can't use the same image for these two devices, so as a workaround,
remove ZR-600DHP from the name for now, so the autoupdater can work and
users aren't confused.

This also takes us past the OpenWrt 15.05 final release.

Config for rootfs and grub is not needed anymore (https://dev.openwrt.org/ticket/18074)

Config file not needed anymore (set implicitly by gluon now)

Avoid empty vars

This reverts commit 6da1677d.

This broke due to a recent OpenWrt update.

Tested-by: @kokel

While the images are the same that are used for the Bullet M, adding these
copies will help users choose the correct image for their device.

These are identical to the TL-WR941ND (and identify as TL-WR941ND
internally).

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.

This default to n now by default

These packages are included by OpenWrt by default on this target and don't
need to be specified manually.

Original patch by @MPW1412

Tested-by: @RubenKelevra

The TL-WR741ND will identify itself as a TL-WR740N/ND as well. This is fine as
the images are identical anyways.

This also fixes the TL-WA830RE v1 model name.