[roll third_party/pigweed] pw_system: initial commit

- Added docs, and empty build files to get started.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/72840

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: f67f054347f262c..e8275d3242a9ee6
Roller-URL: https://ci.chromium.org/b/8828383104496501697
Cq-Cl-Tag: roller-builder:pigweed-sample-project-roller
Cq-Cl-Tag: roller-bid:8828383104496501697
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Ibf844aad45edd505cae527785fa15f42acbc7755
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/sample_project/+/72862
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 f67f054..e8275d3 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit f67f054347f262c226cf3467dba3a2234f68cbae
+Subproject commit e8275d3242a9ee66525d0dc78751b1bbcc1d7241