Allow parallel build via scons -jNN
- Avoid use of chdir= in scons Command()
- Add .gitignore for files generated during build

Change-Id: I256a12da17ac0940a5ded81d2c2dec88abc3ca14
Merge "Allow parallel build via scons -jNN - Avoid use of chdir= in scons Command() - Add .gitignore for files generated during build"