gemini: Fix up image generation

gemini: Fix up image generation

I noticed that the image generator for the Gemini generates
some 10+ MB files for the second (application) partition.
This is just wrong. The first 6144K partition named initramfs
easily fits OpenWRTs squashfs,jffs2 overlayed partitions
with nice headroom for storing configuration files.

Generate a blank partition for the hddapp.tgz file and delete
it after generating the firmware image - when performing
updates manually you just want the "rd.z" file around
anyways.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
(backported from f280bd7fa8423c04028757bbf28284574c7a5bc9)