Support reboot in non-chroot modes using grub and an init script

[Googulator]

We use grub-2.06 as 2.12 has some packaging issues, and can't be built in live-bootstrap for now.

Depends on #418 (can be done separately, but then we'll have to remember to apply the changes to make_bootable.sh)

[Googulator]

This has 2 bugs I've found so far:

• grub.cfg needs to reference $DISK, not $PHYSICAL
• our version of mount (util-linux) still relies on /etc/mtab, rather than /proc/mounts, so after reboot, we don't properly remount auxiliary filesystems - I'm still working on a fix for this

[Googulator]

The new version fixes both issues.

[Googulator]

Note also Stikonas's comments in https://github.com/fosslinux/live-bootstrap/pull/418 (they actually belong here)

[Googulator]

Just issues with pregened files in grub;

* there is a gnulib in `grub-core/lib/gnulib`

* see `autogen.sh` for a bunch of different regenerations that need to take place (unicode, libgcrypt, autogen-ish)

(Side note; the autogen here is WEIRRRD. Doesn't use proper autogen, rather uses some subset they wrote their own Python parser for a subset of autogen??)

Don't worry about the contrib/ stuff, that's not really generated code...

Should be resolved in the last force-push, but I'm still waiting for the build, to see if the checksum changes.

[Googulator]

Fix coming soon, waiting for local build to finish so I can add the final checksum.

[Googulator]

Looks like the checksum was unaffected.

[Googulator]

Let's try again with a small fix to ensure reproducibility between 32-bit and 64-bit host kernels (otherwise a 32-bit userspace on a 64-bit kernel would be detected as "i686-pc-linux-muslx32" vs "i686-pc-linux-musl" on a 32-bit kernel).