Make parallel build work (scons -jNN).
- Mostly eliminate chdir=1 in Command()
- Some minor clean-up in Install() targets to use an explicit Dir().

Change-Id: Ia961fa6a4ea06b61856e3d21bcab7c2359876476
Merge "Make parallel build work (scons -jNN). - Mostly eliminate chdir=1 in Command() - Some minor clean-up in Install() targets to use an explicit Dir()."