gemini: override IB-4220-B partitions for firmware

gemini: override IB-4220-B partitions for firmware

To optimize the flash usage and to make firmware upgrades
simpler, catenate the three firmware partitions "Kern",
"Ramdisk" and "Application" into one, and use all of this
for the combined MTD-splitted kernel+rootfs.

This works fine as long as the kernel is placed in the
beginning of this firmware partition and we leave the
RedBoot partition as is, so the boot loader still can load
the kernel from the first two RedBoot partitions.

Using the RedBoot partitions "as is" can be considered
harmful, because when you flash to a RedBoot partition the
file size is used for downsizing of the partition and make
firmware upgrades fail if they are larger than the RedBoot
partition size after flashing, despite there is actually
flash there. So overriding with fixed partitions is just
generally a good idea.

Link: https://github.com/openwrt/openwrt/pull/21820
Signed-off-by: Linus Walleij <linusw@kernel.org>