ath79: combine UniFi AC dual firmware-partitions

ath79: combine UniFi AC dual firmware-partitions

In order to maximize the available space on UniFi AC boards using a
dual-image partition layout, combine the two OS partitions into a single
partition.

This allows users to access more usable space for additional packages.

Don't limit the usable image size to the size of a single OS partition.
The initial installation has to be done with an older version of OpenWrt
in case the generated image exceeds the space of a single kernel
partition in the future.

Signed-off-by: David Bauer <mail@david-bauer.net>