Expand the usage to be able to specify KERNEL and ROOTFS
on commandline. This helps in using the script for booting
images that are essentially not part of OE-Core
(From OE-Core rev: c985b02130658dd64581ecf14b16e2c70d1d8db5)
Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>