scripts/runqemu: Add support for 'qemumicroblaze' machine

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)

(From OE-Core rev: 2c164a5dfc877d180ef58d46c063573621297929)

Signed-off-by: Nathan Rossi <nathan.rossi@xilinx.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>