support both %runscript and %post-umount to perform scripts before packaging

support both %runscript and %post-umount to perform scripts before packaging

- add %post-umount section which behaves the same as %runscript
  (if both %runscript and %post-umount are defined then KickstartError occurs)
- move post umount scripts from /var/tmp/post_umount_scripts/ on image to host
- post umount scripts can be use UMOUNT_SCRIPTS_PATH env variable
  (UMOUNT_SCRIPTS_PATH: directory path where post umount scripts are located on host)

Change-Id: Ib88c103f0d3e84c067d0d6683f406a5cac92739a
Signed-off-by: Dohyung Kim <dohyung2.kim@samsung.com>