x86: don't build ext4 images

Since we're discarding the ext4 images anyways, we now stop building 
them as well and save a few seconds of build time.

targets/x86.inc

@@ -40,6 +40,9 @@ packages {
 	'kmod-mt7615e',
 }
 
+-- We do not use the ext4 images, so we do not want to build them.
+config('TARGET_ROOTFS_EXT4FS', false)
+
 defaults {
 	factory = '-squashfs-combined',
 	factory_ext = {'.img.gz', '.vmdk', '.vdi'},