This change defines the build target system used by Pigweed, where the
build is configured using per-target globals imported from a target .gni
file. A single build arg specifying the path to the target file replaces
the previous args.
A default build variables file is provided in the Pigweed root. This
file defines and documents all of the build variables used by Pigweed.
A top-level targets directory is added to define the targets used by
Pigweed developers. Currently, only a "host" target is added, which
configures the build for a development system based on the host OS.
Change-Id: Ie62b8d41e5789bfe7fffe20462160142c79076c4
