This is particularly problematic since qemu images switched to ext4 by
default and now cannot work properly with UIs like hob.
This patch adds in ext4 to the appropriate IMAGE* variables fixing this.
[YOCTO #7426]
(From OE-Core rev: e6aab245623b6ba1ca5f06632d1028c863cef737)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>