ramips: fix Airlink AR725W factory image build

ramips: fix Airlink AR725W factory image build

The factory image can't be bigger than 3328 KByte. If the image is
bigger than that, the gemtek-header tool throws an error and breaks
the build.

Make sure the output file to which the gemtek header should be added
exists and wasn't removed during the check-size step because of it
size. This will prevent hard errors in case the factory image is to big
similar to what is done for sysupgrade images.

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