ath79: do not build TP-Link tiny images by default

ath79: do not build TP-Link tiny images by default

For quite some time, the tiny (4M flash) TP-Link sysupgrade and
factory images cannot be built anymore by the buildbots, just
the initramfs-kernel.bin files are still there.

Disable these images for the buildbots and don't waste build
resources.

Note that these devices still build fine with default settings,
just not with the additional packages and config symbols for
the buildbots.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>