[roll third_party/pigweed] kconfig: Add help text

Improve documentation of Kconfig options and prune some unused ones.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/152630

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 698999aa748cb6d..7196e1d7adf4440
Roller-URL: https://ci.chromium.org/b/8777549483689373905
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I981dc66a02640596f0fd9234ae4918f2226d3925
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/152881
Commit-Queue: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Bot-Commit: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index 698999a..7196e1d 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 698999aa748cb6d128070de367690fdaa1dcd697
+Subproject commit 7196e1d7adf4440b15653e308b3e50f453195319