use ext4 image in WKS configuration

[jluebbe]

With the current configuration, WKS will generate new file system images for both rootfs partitions:

meta-rauc-community/meta-rauc-qemux86/wic/qemux86-grub-efi.wks

Lines 15 to 16 in 3d43459

	part / --source rootfs --ondisk sda --fstype=ext4 --label root_a --fixed-size=150M --align 4096
	part / --source rootfs --ondisk sda --fstype=ext4 --label root_b --fixed-size=150M --align 4096

These filesystems will have unnecessary differences to the ext4 generated directly from the image recipe via ext4 in IMAGE_FSTYPES, leading to less reuse during adaptive updates. Instead, the WKS file should reuse the existing ext4 image.