make mail optional

src/func/on_exit

@@ -4,8 +4,9 @@
 
 #on_exit [error_level]
 function on_exit() {
+  ### send mail
+  [ -n "$log_mail_aim" ] && {
     #send log and errorlevel[success/errorr xy]
-
     if [ "$1" != "0" ]; then
       log_mail_subject="$log_mail_subject [ERROR]"
     else
@@ -20,7 +21,9 @@ function on_exit() {
         cat "$log_file"
       } | sendemail -s "$log_mail_smtp" -f "$log_mail_source" -t "$mail_aim" -u "$log_mail_subject" -o tls=no
     done
+  }
 
+  ### clean up
   [ "$1" != "0" ] && {
     chroot_umount$distro "$chroot_path" 2> /dev/null
     workspace_erase "$iso_extr_dir/" "$chroot_path/" 2> /dev/null