ipq806x: Enlarge D7800 flash - use netgear partition

Increase the available flash memory size in Netgear R7800
by repurposing the unused "netgear" partition that is
located after the firmware partition.

Available flash space for kernel+rootfs+overlay increases
by 68 MB from 32 MB to 100 MB.

In a typical build, overlay space increases from 15 to 85,
increasing the package installation possibilities greatly.

Reverting to the OEM firmware is still possible, as the OEM
firmware contains logic to initialise the "netgear" partition
if its contents do not match expectations. In OEM firmware,
"netgear" contains 6 UBI sub-partitions that are defined in
/etc/netgear.cfg and initialisation is done by /etc/preinit

This is based on https://github.com/openwrt/openwrt/commit/fb8a578aa70572b3e56b64d296e22c2931e77b69



Signed-off-by: Mike Lothian <mike@fireburn.co.uk>