firmware-utils: mksercommfw: overhaul image creation

firmware-utils: mksercommfw: overhaul image creation

Move the zip compression into a build recipe. Pad the image using the
existing build recipes as well to remove duplicate functionality

Change the code to append header and footer in two steps. Allow to use a
fixed filename as the netgear update image does.

Use a fixed timestamp within the zip archive to make the images
reproducible.

Due to the changes we are now compatible to the gnu89 c standard used by
default on the buildbots and we don't need to force a more recent
standard anymore.

Beside all changes, the footer still looks wrong in compare to the
netgear update image.

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