Use Git Version of Grub

[wkennington]

The current version of grub2 doesn't have the same level of boot support for btrfs / zfs that the git version does. This patch adds the option to use a development version of grub which tracks a similar version to that of archlinux / gentoo.
Additionally, update the grub configuration scripts to handle more complex setups such as full zfs on root.

[jcumming]

I have an old pull request for adding some automated tests for zfs:

NixOS/nixos#246

I ... uh ... gave up when it came time to hack on grub.

[wkennington]

Sounds good, I was going to make some tests myself after I was satisfied I could get the initrd into working order. The problem I'm having right now is that the initrd isn't waiting long enough for my disks to come up so it fails to import the zpool. There is also the issue of the spl hostid but I'll get around to that after. Otherwise, a full on zfs setup works for / without a separate boot now.

I'm focusing on getting the os-specific/linux packages more up to date.
Change the initrd to use systemd instead of a basic shell script.
In the process create proper lvm / zpool / btrfs detection.

Afterward we should have pretty good filesystem support at boot.