overlay/ignition: delete pre-existing bootuuid.cfg

In our disk images, we inject a grub script that looks for a
`/boot/grub2/bootuuid.cfg` to find the boot partition. If the file is not
present, the label `boot` is used to find the partition. See [1] for more.

During the first boot, the hard-coded uuid of the boot partition is
replaced with a random one [2]. However this is triggered by checking
for the `bootuuid.cfg` file, and this file is created by `bootc install`,
short-circuiting this logic.

So let's delete this file pre-emptyvely for during the first boot, so we
always restamp the boot partition and write a new file in /boot.
This allows us to use `bootc install to-disk` to build our disk images.
See [3][4].

[1] https://github.com/coreos/bootupd/blob/1768f31c85b5d8cd72f4339ab669b703d7071b51/src/grub2/grub-static-pre.cfg
[2] https://github.com/coreos/fedora-coreos-config/blob/testing-devel/overlay.d/05core/usr/lib/dracut/modules.d/35coreos-ignition/coreos-gpt-setup.service
[3] https://issues.redhat.com/browse/COS-3374
[4] coreos/coreos-assembler#4224

Author: jbtrystram

overlay.d/05core/usr/lib/dracut/modules.d/35coreos-ignition/coreos-gpt-setup.service

@@ -28,3 +28,6 @@ OnFailureJobMode=isolate
 Type=oneshot
 RemainAfterExit=yes
 ExecStart=/usr/sbin/coreos-gpt-setup /dev/disk/by-label/boot
+# MountFlags=slave is so the umount of /boot is guaranteed to happen.
+# /boot will only be mounted for the lifetime of the unit.
+MountFlags=slave

overlay.d/05core/usr/lib/dracut/modules.d/35coreos-ignition/coreos-gpt-setup.sh

@@ -26,6 +26,19 @@ if [ "${PTUUID:-}" != "$UNINITIALIZED_GUID" ]; then
     exit 0
 fi
 
+
+# Mount /boot. Note that we mount /boot but we don't unmount it because we
+# are run in a systemd unit with MountFlags=slave so it is unmounted for us.
+bootmnt=/mnt/boot_partition/
+bootdev=/dev/disk/by-label/boot
+mount -o rw ${bootdev} ${bootmnt}
+# Make sure we delete any pre-existing bootuuid.cfg
+# bootc install stamps it during the disk image creation
+# since we are changing the uuid here we want to remove this file
+# otherwise it will prevent a reboot
+rm -f ${bootmnt}/grub2/bootuuid.cfg
+
+
 echo "Randomizing disk GUID"
 sgdisk --disk-guid=R --move-second-header "$PKNAME"
 udevadm settle || :