combined-ext-image.sh: generate image in temp dir

combined-ext-image.sh: generate image in temp dir

The new build commands operate on the input image and use it again as
output image. This conflicts with the way combined-ext-image.sh was
operating. It required that input and output files are different files and
and that it can write freely to the output file.

This can be avoided when all intermediate build steps by
combined-ext-image.sh are done in a temporary directory. The output file is
then only overwritten in the last step.

Signed-off-by: Sven Eckelmann <sven.eckelmann@open-mesh.com>