Generate version information for RAUC when rauc.db is empty (#3436)

RAUC currently doesn't know the version of the booted slot when booted for the
first time or after wiping the data partition. As a result `ha os info` is
missing this information too.

As there's no built-in mechanism for generating these data by RAUC itself, add
a oneshot service that checks if the boot slot information is contained in the
rauc.db and if not, then generate it.

RAUC seems to cope quite well even with bogus data contained in rauc.db but in
any case, a test has been added to check that everything works as expected.

buildroot-external/rootfs-overlay/usr/libexec/raucdb-update

@@ -0,0 +1,39 @@
+#!/bin/sh
+# shellcheck disable=SC1091
+
+set -e
+
+if grep -q 'slot\.boot\.0' /mnt/data/rauc.db; then
+    echo "[INFO] rauc.db already contains slot information"
+    exit 0
+fi
+
+echo "[INFO] Generating rauc.db from os-release data"
+
+eval "$(rauc status --output-format=shell)"
+
+if [ -z "${RAUC_SYSTEM_BOOTED_BOOTNAME}" ]; then
+    echo "[ERROR] RAUC_SYSTEM_BOOTED_BOOTNAME is empty"
+    exit 1
+fi
+
+CURRENT_SLOT_ID=$(test "${RAUC_SYSTEM_BOOTED_BOOTNAME}" = "A" && echo 0 || echo 1)
+
+. /etc/os-release
+
+cat >> /mnt/data/rauc.db <<EOF
+
+[slot.boot.0]
+bundle.compatible=${RAUC_SYSTEM_COMPATIBLE}
+bundle.version=${VERSION_ID}
+
+[slot.kernel.${CURRENT_SLOT_ID}]
+bundle.compatible=${RAUC_SYSTEM_COMPATIBLE}
+bundle.version=${VERSION_ID}
+
+[slot.rootfs.${CURRENT_SLOT_ID}]
+bundle.compatible=${RAUC_SYSTEM_COMPATIBLE}
+bundle.version=${VERSION_ID}
+EOF
+
+/usr/bin/systemctl restart rauc.service