gemini: add wiligear image build code

gemini: add wiligear image build code

They were dropped with 6409b159e8b8 ("gemini: switch to 4.14") without
any explaination.

The image generation is disabled for now as it would break the build for
the target. The mkfwimage2 call need to be adjusted to reflect the real
size of kernel and rootfs. Nevertheless, add the required code to give
interested parties a chance to fix the remaining issues.

The dts would need to use the ecoscentric,redboot-fis-partitions
partition parser to get the correct partition offsets and size. It's
expected that the OEM firmware adjusts the on flash partition table with
the values defined in the image header.

Signed-off-by: Mathias Kresin <dev@kresin.me>