Enable /boot on btrfs subvolume with GRUB2

This depends on a version of grubby with btrfs subvolume support. For
example, grubby >= 8.40-10.fc29.

This essentially reverses the block imposed in response to rhbz#864198
and correctly resolves rhbz#1038885.

anaconda.spec.in

@@ -131,6 +131,10 @@ Requires: glibc-langpack-en
 # even though the distro default is dbus-broker in F30+
 Requires: dbus-daemon
 
+# Ensure it's not possible for a version of grubby to be installed
+# that doesn't work with btrfs subvolumes correctly...
+Conflicts: grubby < 8.40-10
+
 Obsoletes: anaconda-images <= 10
 Provides: anaconda-images = %{version}-%{release}
 Obsoletes: anaconda-runtime < %{version}-%{release}

pyanaconda/bootloader/grub2.py

@@ -114,7 +114,7 @@ class GRUB2(BootLoader):
     stage2_must_be_primary = False
 
     # requirements for boot devices
-    stage2_device_types = ["partition", "mdarray"]
+    stage2_device_types = ["partition", "mdarray", "btrfs subvolume"]
     stage2_raid_levels = [raid.RAID0, raid.RAID1, raid.RAID4,
                           raid.RAID5, raid.RAID6, raid.RAID10]
     stage2_raid_member_types = ["partition"]