script/unit-test: Refactor out individual build
script/unit-test: Refactor out individual build

This doesn't change any behavior, but it will allow us to perform builds
outside of the original install_deps flow.

Change-Id: I93b6bca86ed92e28fde6c1503c28f123bf8c3da4
Signed-off-by: William A. Kennington III <30fff5198f4d10ca1117e5f3ec82f3b4b8900d81@google.com>

openbmc/openbmc-build-scripts,openbmc/openbmc-build-scripts