runqemu: Allow the user to override the device tree option
Update the runqemu script to allow the user to specify a device tree
to boot when calling runqemu.
This involves creating a seperate check_dtb() function incase the user
has specified 'none' for the kernel but still wants a device tree.
Signed-off-by: Alistair Francis <alistair.francis@xilinx.com> Reviewed-by: Ben Levinsky <ben.levinsky@xilinx.com> Cc: Ben Levinsky <ben.levinsky@xilinx.com> Signed-off-by: Ross Burton <ross.burton@intel.com>
