Refactor documentation build rules to expose attributes

The gerrit-review build runs on something similar to Buck, but
doesn't use the BUCK file. Extract the attribute configuration that
used to be in the shared asciidoc.conf file into a .defs file that
can be imported by both build systems.

Hoist the git_version() up into the BUCK file, allowing either build
system to supply the Git revision and keep the definition shared.

Change-Id: I3429f966d0d8debb7b23a3b5142763c3ca4ba8e2
3 files changed