ipq806x: Enlarge R7800 flash - use netgear partition

ipq806x: Enlarge R7800 flash - use netgear partition

Increase the available flash memory size in Netgear R7800
by taking into the use 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

Tested with Openwrt master r7093-4fdc6ca31b and OEM V1.0.2.52

Reference to forum discussion in Netgear R7800 exploration thread:
https://forum.lede-project.org/t/netgear-r7800-exploration-ipq8065-qca9984/285/1118
(messages 1118-1158)

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>