Use verity bundle format for RAUC updates (#3152)

We originally enabled it in 0ebcdcb9dc
but later reverted the patch in 5b927389b8
because of backward compatibility issues. Since we're going to 12.0,
there is now hopefully enough room for seamless transition.

Co-authored-by: Stefan Agner <stefan@agner.ch>

buildroot-external/ota/manifest.raucm.gtpl

@@ -2,6 +2,9 @@
 compatible={{ env "ota_compatible" }}
 version={{ env "ota_version" }}
 
+[bundle]
+format=verity
+
 [hooks]
 filename=hook
 hooks=install-check;