x86: fix missing squashfs and ext4 rootfs images

The previous rework of x86 image creation done in commit cb007a7bf6
("x86: switch image generation to new code") removed images of separate
squashfs and ext4 filesystems which are handy for example in testing
under QEMU.

So this patch adds back creation of those missing rootfs images for ext4
and squashfs based filesystems.

Fixes: cb007a7bf6 ("x86: switch image generation to new code")
Signed-off-by: Paul Spooren <mail@aparcar.org>
[commit subject and description tweaks]
Signed-off-by: Petr Štetiar <ynezz@true.cz>

target/linux/x86/image/Makefile

@@ -99,10 +99,12 @@ define Device/Default
   IMAGE/combined.img.gz := grub-config pc | combined | grub-install | gzip
   IMAGE/combined.vdi := grub-config pc | combined | grub-install | qemu-image vdi
   IMAGE/combined.vmdk := grub-config pc | combined | grub-install | qemu-image vmdk
+  IMAGE/rootfs.img := append-rootfs
+  IMAGE/rootfs.img.gz := append-rootfs | gzip
   ifeq ($(CONFIG_TARGET_IMAGES_GZIP),y)
-    IMAGES := combined.img.gz
+    IMAGES := combined.img.gz rootfs.img.gz
   else
-    IMAGES := combined.img
+    IMAGES := combined.img rootfs.img
   endif
   KERNEL := kernel-bin
   KERNEL_INSTALL := 1