Remove existing rauc.db from a data disk on the first boot

If data disk is adopted on Yellow using the mechanism added in #3686, it
contains RAUC version information that is very likely invalid. In such case,
remove the file on first boot and have it recreated by the raucdb-update
service.

buildroot-external/rootfs-overlay/usr/lib/systemd/system/raucdb-first-boot.service

@@ -0,0 +1,14 @@
+[Unit]
+Description=Remove adopted rauc.db on first boot
+After=rauc.service
+Before=raucdb-update.service
+RequiresMountsFor=/mnt/data
+ConditionFirstBoot=yes
+ConditionFileNotEmpty=/mnt/data/rauc.db
+
+[Service]
+Type=oneshot
+ExecStart=/bin/rm -f /mnt/data/rauc.db
+
+[Install]
+WantedBy=multi-user.target