Updated image creation instructions. Used wic images instead
of manually putting rootfs and kernel to SD card.

[YOCTO #8719]

(From meta-yocto rev: 2b030a6061a33334bec63fce23447cbbe1846c86)

Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>