x86: switch to EFI-compatible images

The main difference between the non-EFI and EFI images generated by
OpenWrt is that the former uses an MS-DOS partition table, while the
latter uses GPT. The EFI images still have a BIOS-compatible MBR, so
they work fine on non-EFI systems.

Closes #2403

targets/x86.inc

@@ -44,8 +44,8 @@ packages {
 config('TARGET_ROOTFS_EXT4FS', false)
 
 defaults {
-	factory = '-squashfs-combined',
+	factory = '-squashfs-combined-efi',
 	factory_ext = {'.img.gz', '.vmdk', '.vdi'},
-	sysupgrade = '-squashfs-combined',
+	sysupgrade = '-squashfs-combined-efi',
 	sysupgrade_ext = '.img.gz',
 }