[archiso] Avoid regenerate checksum -> .iso.

Also put _is_directory_changed() inside 'if' avoiding premature execution if it returns non-zero.

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

archiso/mkarchiso

@@ -293,11 +293,13 @@ _mkfs () {
 command_checksum () {
     _show_config checksum
 
+    if _is_directory_changed "${work_dir}/iso/${install_dir}" "${work_dir}/iso/${install_dir}/checksum.md5"; then
         _msg_info "Creating checksum file for self-test"
         cd "${work_dir}/iso/${install_dir}"
         find -type f ! -name checksum.md5 -print0 | xargs -0 md5sum > checksum.md5
         cd ${OLDPWD}
         _msg_info "Done!"
+    fi
 }
 
 # Create an ISO9660 filesystem from "iso" directory.
@@ -308,8 +310,7 @@ command_iso () {
 
     _show_config iso
 
-    _is_directory_changed "${work_dir}/iso" "${img_name}"
-
+    if _is_directory_changed "${work_dir}/iso" "${img_name}"; then
         _msg_info "Creating ISO image..."
         local _qflag=""
         if [[ ${quiet} == "y" ]]; then
@@ -327,6 +328,7 @@ command_iso () {
             -o "${img_name}" "${work_dir}/iso/"
         isohybrid "${img_name}"
         _msg_info "Done! | $(ls -sh ${img_name})"
+    fi
 }
 
 # Parse aitab and create each filesystem specified on that, and push it in "iso" directory.