scripts/runqemu: Add support for 'qemumicroblaze' machine
* Add support to boot the 'qemumicroblaze' machine in
qemu-system-microblazeel
* Use the specific machine model for a MicroBlaze system 'petalogix-ml605'
* Use the DTB generated from the kernel build as the DTB for boot
* Force use of initrd rootfs (either in ext or cpio formats)