[core] add FreeNAS support

* And reverted grub2 to the one from b3947fc0263f2301d29d26742539751ba2cc1b28 (reverts commit 8b47e95eb5cc2ca1fe989903e5620f82d0b32ab9)
  since FreeNAS doesn't work with the older one.
  Sorry "Super Grub2 Disk" and other older GRUB2 based tools, but you'll need to
  update the Grub version you use if you want to be compatible with Rufus.
* Closes #244