Check if there are any modification of files/directories inside
the target directory for squashfs image.

Implements in some way this item in TODO:
* Add 'needsupdate' function to check if a squashfs image in the
  iso dir is up to date, if so, skip it; else rebuild it. No more
  need for the -f flag

Signed-off-by: Gerardo Exequiel Pozzi <vmlinuz386@yahoo.com.ar>