Add build.py, build step to yaml and metadata_setup.

This adds the implementation of the metadata setup stepwith unit tests.
Introduces the python module inspect.cleandoc to the script.

Tested: I ran unit tests with pytest to test for function behaviors
of git_metadata step. All test cases passed. I also ran an end-to-end
test with the sstate-cache, build was successful.

Signed-off-by: Simon-Lii <thesimonli@google.com>
Change-Id: I6f85a58524e036a00b9c7c237f1013872b269d83