runqemu: Allow the user to specity no kernel or rootFS
In some cirsumstances the user doesn't want to supply a kernel, rootFS
or DTB to QEMU. This will occur more now that QEMU supports loading
images using a '-device loader' method.
Allow users to specify 'none' for QB_DEFAULT_FSTYPE or QB_DEFAULT_KERNEL
to avoid supplying these options to QEMU.
Signed-off-by: Alistair Francis <alistair.francis@xilinx.com> Signed-off-by: Ross Burton <ross.burton@intel.com>
