roll: third_party/pigweed: pw_build: Add flags_from_dict

Break out the flag formatting functionality from merge_flags.
This is actually a more natural API.

Original-Bug: 301334234
Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/221691
Lint: Lint 🤖 <android-build-ayeaye@system.gserviceaccount.com>

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 73ef002b92f1710..97aecb2d19d2df0
Roller-URL: https://ci.chromium.org/b/8742754176439190289
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Ifc60e76babdb3b5c87fdc9ad2dfda76b8149137c
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/examples/+/221295
Bot-Commit: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Commit-Queue: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Lint: Lint 🤖 <android-build-ayeaye@system.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index 73ef002..97aecb2 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 73ef002b92f1710537635fdb38c59dc1073d025e
+Subproject commit 97aecb2d19d2df0a05b75b29f72848290d9bc388