ath79: disable default build for devices with 4M flash

ath79: disable default build for devices with 4M flash

It has been decided that the 19.07 release will be last one to include
4/32 devices.

This disables default build for the remaining devices with 4M flash
on ath79. Note that this will leave exactly one enabled device for
ath79/tiny subtarget, PQI Air-Pen, which was moved there due to
kernel size restrictions.

All 4M TP-Link devices have already been disabled in
8819faff47ff ("ath79: do not build TP-Link tiny images by default")

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