Cache as we go instead of in the end

Cache as we go instead of in the end

In order to speed up the build process when building with the cache
enabled, cache immediately following the build of a sub artifact
instead of at the end.

Should the build fail, we don't need to rebuild those parts that
already were successful in the last build.

Change-Id: I23b59f7ba4a04272aea855cf451793158fe12241
Signed-off-by: Stefan K. Berg <stefan.k.berg@ericsson.com>