malta: fix overlay support by updating to f2fs/ext4 based overlays

malta: fix overlay support by updating to f2fs/ext4 based overlays

Current snapshot images doesn't provide images with usable overlay
filesystems:

 mount_root: no usable overlay filesystem found, using tmpfs overlay

So this patch fixes this by updating support to current f2fs/ext4 based
overlays and providing squashfs with proper rootfs
TARGET_ROOTFS_PARTSIZE size.

While at it, I've also added support for optional gzipping of the
images, and renamed the output rootfs image filenames to match the more
common naming pattern $(IMG_PREFIX)-rootfs-$(FS-TYPE).img.

Signed-off-by: Petr Štetiar <ynezz@true.cz>