* Add dependency on pigweed

Pigweed is a open source project containing libraries and tools for
embedded development. It provides a package management system that
downloads versioned development tools including ARM GCC, cmake, GN,
ninja, openocd, and python on Linux, Mac, & Windows. These packages are
hosted by Google in CIPD, which is part of Chrome's open source
infrastructure.

Using the tools provided by pigweed is not required, but having the
option of using a reproducible development environment is useful,
especially for new developers.

Usage:
  source bootstrap.sh  # first time and after an update
  source activate.sh  # setup environment from prior bootstrap

* Add CHIP branding to bootstrap

Co-authored-by: Keir Mierle <keir@google.com>