rootfs.py: find .ko.gz and .ko.xz kernel modules as well

* with xz PACKAGECONFIG enabled in kmod and xz module compression enabled in kernel
  the do_rootfs task doesn't run depmod in the image, because it thinks there are no modules:
  NOTE: No Kernel Modules found, not running depmod

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Christophe Chapuis <chris.chapuis@gmail.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>