osbase/build/mkverity: generate salt from product info

The salt is now generated from the product info file, instead of the
input image file. This reduces build time by around 0.3 s.

Change-Id: Id3263e24604745324a5652658ff79cc79c9df5fa
Reviewed-on: https://review.monogon.dev/c/monogon/+/4431
Reviewed-by: Lorenz Brun <lorenz@monogon.tech>
Tested-by: Jenkins CI
diff --git a/metropolis/node/BUILD.bazel b/metropolis/node/BUILD.bazel
index fd6dbcc..bdf7d8c 100644
--- a/metropolis/node/BUILD.bazel
+++ b/metropolis/node/BUILD.bazel
@@ -117,6 +117,7 @@
 
 verity_image(
     name = "verity_rootfs",
+    salt = ":product_info",
     source = ":rootfs",
 )