efi: Don't display a uefi-firmware entry if it's not supported

Add a new --is-supported option to commands/efi/efifwsetup and
conditionalize display on it.

Signed-off-by: Robbie Harwood <rharwood@redhat.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

util/grub.d/30_uefi-firmware.in

@@ -31,7 +31,8 @@ LABEL="UEFI Firmware Settings"
 gettext_printf "Adding boot menu entry for UEFI Firmware Settings ...\n" >&2
 
 cat << EOF
-if [ "\$grub_platform" = "efi" ]; then
+fwsetup --is-supported
+if [ "\$grub_platform" = "efi" -a "\$?" = 0 ]; then
 	menuentry '$LABEL' \$menuentry_id_option 'uefi-firmware' {
 		fwsetup
 	}