build-unit-test-docker: automatically lookup HEAD rev
build-unit-test-docker: automatically lookup HEAD rev

Switch from a special 'head_pkgs' list to automatically looking
up the HEAD revision of any package that doesn't have a specified
revision.  Also, switch the 'pkg_rev' to a dictionary so we can add
additional fields later on.

Signed-off-by: Patrick Williams <cbb7353e6d953ef360baf960c122346276c6e320@stwcx.xyz>
Change-Id: I8ead996469966f087845c6b3cbd6eb5226f3e235

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