libcr51sign: build: Refactor for subproject use

This refactors the build to make it more suitable for use as a
subproject in other OpenBMC codebases.

Change-Id: I546c993d3f53c1cbe2161e5d8959373d5b12e57f
Signed-off-by: William A. Kennington III <wak@google.com>
8 files changed