runqemu: Honor KERNEL and ROOTFS settings from environment

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>

scripts/runqemu

@@ -54,9 +54,9 @@ error() {
 }
 
 MACHINE=${MACHINE:=""}
-KERNEL=""
+KERNEL=${KERNEL:=""}
+ROOTFS=${ROOTFS:=""}
 FSTYPE=""
-ROOTFS=""
 LAZY_ROOTFS=""
 SCRIPT_QEMU_OPT=""
 SCRIPT_QEMU_EXTRA_OPT=""