Move BUILD.gn files into submodules

Moving two files:
  driver/BUILD.gn -> driver/linux/BUILD.gn
  third_party/BUILD.gn -> third_party/googletest/BUILD.gn

GN sets $target_out_dir according to the location of the build file.
These two target therefore used to build inside the parent directory
rather than the directory corresponding to the location of their source
files. Moving them avoids potential conflict with other/future build
targets.

This is also useful because it eliminates the need to symlink these
files in repo-based builds.

Change-Id: Icc992c2e258e11afe39e2530afe198fa2d62905b
7 files changed