Set -u in live-snapshot only in debug mode.

* Although is really fine to have checks for unboud variables, this
  limits also included code, where unbound variables could be used as
  feature. Set LIVE_SNAPSHOT_CHECK_UNBOUND to have it on again in your
  test runs.

Signed-off-by: Marco Amadori <amadorim@vdavda.com>

bin/live-snapshot

@@ -30,7 +30,12 @@
 ROOTSNAP=""
 HOMESNAP=""
 
-set -eu
+if [ -n "${LIVE_SNAPSHOT_CHECK_UNBOUND}" ]
+then
+	set -eu
+else
+	set -e
+fi
 
 . /usr/share/initramfs-tools/scripts/live-helpers