For regular use, a full disk image is always recommended, as it is
required to support sysupgrades.

During development or for automated tests, separate images for the
kernel and rootfs may be useful to pass additional kernel cmdline or use
nfsroot/virtiofs. The rootfs is only available as a (squashfs)
filesystem image, not as a TAR archive (the TAR archive in OpenWrt's bin
directory does not contain DEVICE_PACKAGES, so it is missing most of
Gluon's packages).

The old bash-based parsing code was way too complex. Replace it with Lua.

Signed-off-by: Petr Štetiar <ynezz@true.cz>

This allows for smaller firmware images and makes it possible to reset a
device via firstboot command.

Signed-off-by: Matthias Fritzsche <matthias@chemnitz.freifunk.net>

Let's keep the default package selection to hardware support.

Fixes #1022

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

This also takes us past the OpenWrt 15.05 final release.

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.