[roll third_party/pigweed] pw_software_update: Verify root metadata signature

Original-Bug: 456
Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/67042

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: a1d6cd641276fe1..28072eb8427f2da
Roller-URL: https://ci.chromium.org/b/8828181982368819201
Cq-Cl-Tag: roller-builder:pigweed-sample-project-roller
Cq-Cl-Tag: roller-bid:8828181982368819201
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Ic364544054dfcd57201c387b7a50e61a9786fa6b
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/sample_project/+/73580
Bot-Commit: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
Commit-Queue: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index a1d6cd6..28072eb 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit a1d6cd641276fe172249e4e15069daef6dc5a8ac
+Subproject commit 28072eb8427f2dac1c43fdfe7532daf690aaaa00