disk-image.binary: actually skip building MBR image on EFI platforms.

The case is for arch:subarch combo, not just arch alone even if
subarch is empty. Thus currently on adm64/arm64/armhf ubuntu-cpc
builds mbr image is created and then ignored, as the convert to qcow2
hook prefers the uefi image whenever available.

Skipping building these correctly, should speed up the build a little
bit and use slightly less disk space.

live-build/ubuntu-cpc/hooks.d/base/disk-image.binary

@@ -19,10 +19,6 @@ case $ARCH:$SUBARCH in
 		echo "POWER disk images are handled separately"
 		exit 0
 		;;
-	amd64|arm64|armhf)
-		echo "We only create EFI images for $ARCH."
-		exit 0
-		;;
 	armhf:raspi2)
 		# matches the size of the snappy image
 		IMAGE_SIZE=$((4*1000*1000*1000))
@@ -31,6 +27,10 @@ case $ARCH:$SUBARCH in
 		BOOTPART_END=138M
 		BOOT_MOUNTPOINT=/boot/firmware
 		;;
+	amd64:*|arm64:*|armhf:*)
+		echo "We only create EFI images for $ARCH."
+		exit 0
+		;;
 	*)
 		;;
 esac