Require targets to explicitly import pigweed variable definitions. This
moves out some necessary logic from the BUILDCONFIG.gn file.
Change-Id: If9c9fe5a680cc9d897a0e8a55889873be618ed84
This CL reworks toolchains to be defined as scopes composing configs and
toolchain args. This allows individual targets to add configs to or
remove configs from the defaults.
Change-Id: I105a231f3e2de35499e1617241f3eeb42ef3248d