Remove yum cache to reduce image size

mkimage-yum.sh has a lot in common with mkimage-rinse.sh, including extra
files clean up. Sync with mkimage-rinse.sh by removing yum cache to reduce
the image size.

Minor changes to reduce also the delta/diff and make it easier to sync with
mkimage-rinse.sh script.

Signed-off-by: Fathi Boudra <fathi.boudra@linaro.org>