build: Add /boot/.coreos-assembler-buildid to target

It will be very useful in the future to be able to more rigorously
know the build that a given machine *started* from.  For example,
small tweaks like `chattr +i /sysroot` are things that won't
happen for in-place updates.

If we decide to introduce a mechanism (e.g. systemd
unit) that performs those changes even for old in-place installs,
it could be useful to know exactly what the starting state was.

I chose `/boot` since it's a relatively fixed location which
people generally won't wipe and replace without also erasing
everything else.

src/cmd-buildextend-metal

@@ -173,7 +173,7 @@ if [ -z "${use_anaconda}" ]; then
         ref_arg=${commit}
     fi
 
-    runvm -drive "if=virtio,id=target,format=${image_format},file=${path}.tmp" -- /usr/lib/coreos-assembler/create_disk.sh /dev/vda "$ostree_repo" "${ref_arg}" "${ostree_remote}" /usr/lib/coreos-assembler/grub.cfg "$name" "${save_var_subdirs}" "\"$kargs\""
+    runvm -drive "if=virtio,id=target,format=${image_format},file=${path}.tmp" -- /usr/lib/coreos-assembler/create_disk.sh /dev/vda "${img}" "$ostree_repo" "${ref_arg}" "${ostree_remote}" /usr/lib/coreos-assembler/grub.cfg "$name" "${save_var_subdirs}" "\"$kargs\""
     mv "${path}.tmp" "$path"
     echo "{}" > tmp/vm-iso-checksum.json
 else

src/create_disk.sh

@@ -10,8 +10,8 @@
 # an issue and we can discuss configuration needs.
 set -euo pipefail
 
-if [ "$#" -ne 8 ]; then
-	echo 'create_disk <device> <ostree-repo> <ostree-ref> <ostree-remote> <grub-script> <os-name> <save-var-subdirs> <space separated kargs>'
+if [ "$#" -ne 9 ]; then
+	echo 'create_disk <device> <buildid> <ostree-repo> <ostree-ref> <ostree-remote> <grub-script> <os-name> <save-var-subdirs> <space separated kargs>'
 	exit 1
 fi
 
@@ -20,6 +20,7 @@ export PATH=$PATH:/sbin:/usr/sbin
 arch="$(uname -m)"
 
 disk="$1" && shift
+buildid="$1" && shift
 ostree="$1" && shift
 ref="$1" && shift
 remote_name="$1" && shift
@@ -58,6 +59,7 @@ mkdir rootfs/boot
 chcon $(matchpathcon -n /boot) rootfs/boot
 mount "${disk}1" rootfs/boot
 chcon $(matchpathcon -n /boot) rootfs/boot
+echo "${buildid}" > rootfs/boot/.coreos-assembler-buildid
 mkdir rootfs/boot/efi
 # FAT doesn't support SELinux labeling, it uses "genfscon", so we
 # don't need to give it a label manually.