Require targets to explicitly import pigweed variable definitions. This
moves out some necessary logic from the BUILDCONFIG.gn file.
Change-Id: If9c9fe5a680cc9d897a0e8a55889873be618ed84
Introduces the lm3s6965evb target designed to run inside QEMU. Not all
tests have been verified on this target, but the target builds and
successfully runs some of the tests.
Change-Id: Ic0c309c691096aa18b03bd85457195d6a2b49a0b