This has a few advantages:
- We can track dependencies more precisely
- ninja can assemble things in parallel as they're ready rather than
sequentially all at once.
- It is easier to customize SDKs depending on target platform, e.g.
Fuchsia.
This CL also has a number of cleanups:
- Defining is_fuchsia and is_fuchsia host so we don't always have to check
- Piping through toolchain overrides in more places
- Fixing bugs in copy_tree.py, not using list_files.py, which is broken on Windows
related #29558R=whesse@google.com
Review-Url: https://codereview.chromium.org/2848943003 .