mkosi: provide detached verity signatures too for minimal images

Useful for manual testing in the VM

mkosi/mkosi.postinst.chroot

@@ -28,6 +28,7 @@ rm -f /etc/resolv.conf
 
 for f in "$BUILDROOT"/usr/share/*.verity.sig; do
     jq --join-output '.rootHash' "$f" >"${f%.verity.sig}.roothash"
+    jq --join-output '.signature' "$f" | base64 --decode >"${f%.verity.sig}.roothash.p7s"
 done
 
 # We want /var/log/journal to be created on first boot so it can be created with the right chattr settings by