ar71xx: clean up SEAMA image build code

ar71xx: clean up SEAMA image build code

Split seama-factory and seama-sysupgrade into smaller pieces (similar to
the ramips code) to make the image generation more flexible.

Also use standard pad-offset instead of adding a block of zeros at the
beginning of the image that is later cut off again. Standard pad-rootfs can
be used as the seal header doesn't contain an image size or checksum.

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>