Add build_image step.

This adds the implementation of the build_image step with unit tests.

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: I3b0d2b2125973419a638eee70dfff5ec039a280f