Export osguard root hash for signing (#14468)

Adds usr verity root hash export into the OSGuard image definition. This is needed for signing of said root hash.

Author: jiria

toolkit/imageconfigs/osguard-amd64.yaml

@@ -35,6 +35,7 @@ storage:
       hashDeviceId: usr-hash-a
       dataDeviceMountIdType: uuid
       hashDeviceMountIdType: uuid
+      hashSignaturePath: /boot/usr.hash.sig
 
   filesystems:
     - deviceId: esp