openwrt: backport combination of dual-flash partitions

Backport two patches combining dual-flash layouts of OCEDO as well as
UniFi AC boards.

The two firmware partitions are already combined on OpenWrt master to
prolong the life of these devices. It allows the device to store
firmware images up to 14 MB compared to the previous 7 MB.

The intention behind backporting these patches is to allow these devices
to have a wide update path to firmware-versions requiring this extra
space. Otherwise a device might not be able to install an upgrade which
exceeds a single firmware-partition.

For UniFi AC boards it should be noted that factory-installation will
not be possible with an image exceeding a single firmware-partition. In
this case, an older OpenWrt image that fits in a single partition and
supports writing the enlarged partition space is required.

Currently, this is not the case. As these devices are end-of-sale, this
will become less of a concern over time.

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