ath25: disable image generation for ubnt2 and ubnt5 devices

ath25: disable image generation for ubnt2 and ubnt5 devices

Image generations is currently failing on builbots due to the following
errors:

 mkfwimage -B XS2 -v XS2 [...] ath25-ubnt2-squashfs-sysupgrade.bin
 ERROR: Failed creating firmware layout description - error code: -2

 mkfwimage -B XS5 -v XS5 [...] ath25-ubnt5-squashfs-sysupgrade.bin
 ERROR: Failed creating firmware layout description - error code: -2

That cryptic -2 error simply means, that kernel+rootfs doesn't fit into
the firmware partition.

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