Revert "Revert "Merge pull request #2449 from wkennington/master.grub""

This reverts commit 94205f5f21c4d9942bb4205c06229438051b6853.

Conflicts:
	nixos/modules/system/boot/loader/grub/install-grub.pl
nixos/install-grub: Store path should be /nix/store not /nix
nixos/generate-config: Support detecting nested subvolumes
nixos/install-grub: Detect nested btrfs subvolumes
nixos/tests-installer: Add a test for btrfs default volumes and bind mounts
nixos/generate-config: Don't interpret btrfs subvols as bind mounts
nixos/installer-test: Use nested subvolumes for root to test detection
nixos/install-grub: Always copy kernels for different devices
nixos/install-grub: Only check for /nix to be the mountPoint for the store
nixos/generate-config: Only add store search path when kernels are not copied
Merge pull request #3903 from wkennington/master.grub

Bring back grub update with fixes