Create starting code for openbmc gcb ci development

This adds some of the prebuild step. This includes the parent
class Step and the implementation of the child class GitMetadata.

Also adds the prebuild main function and the prebuild portion of the
yaml configuration file.

Signed-off-by: Simon-Lii <thesimonli@google.com>
Change-Id: Ibb7e5482114361c58bbc238fd3762bf73ecb61fa