98-clean-chroot: check existence of updatedb binary via $ROOTCMD.

/usr/bin/updatedb might point to /etc/alternatives/updatedb which
points to another updatedb binary. When testing for precense of
/usr/bin/updatedb the test might fail because test(1) dereferences
symbolic links. So make sure to check existence of updatedb
binary inside $target using $ROOTCMD.

etc/grml/fai/config/scripts/GRMLBASE/98-clean-chroot

@@ -198,7 +198,7 @@ else
 EOF
 fi
 
-if ! [ -x $target/usr/bin/updatedb ] ; then
+if ! $ROOTCMD test -x /usr/bin/updatedb ; then
   echo "Warning: updatedb not installed"
 else
   echo "Updating locate-database"