Running the script on a hddimg when using e.g. meta-minnow will create a grub2 config without a root= cmdline causing the boot to fail.
commit c38da2675bfd0c0c4e78cf65ef48295c75f6f16e Author: Darren Hart <dvhart@linux.intel.com> Date: Tue Mar 19 15:15:02 2013 -0700 mkefidisk: Always specify a root= kernel parameter