image-rpi64: Add firmware partition to /etc/fstab

The firmware partition is required to be permanently mounted at
/boot/firmware by the raspi-firmware package, which otherwise fails
during updates.

Signed-off-by: Ariel D'Alessandro <ariel.dalessandro@collabora.com>

image-rpi64.yaml

@@ -34,7 +34,7 @@ actions:
         options: [ x-systemd.automount ]
       - mountpoint: /boot/firmware
         partition: firmware
-        buildtime: true
+        options: [ x-systemd.automount ]
       - mountpoint: /home
         partition: general_storage