[yocto] Dependencies on other file system types for custom file system type

[Eric Grunt]

> I think there is some translation code somewhere which probably
> confuses things more as some bits have - and some have _. Does the
> squashfs_xz image type actually work?

There is no squashfs_xz image type.
squashfs-xz (by poky) works,
squashfs_xz_ubi (custom type) also works,
but needs to occasionally call IMAGE_CMD_squashfs-xz itself.