Add more BUILD_BROKEN_* logic

Make the values tristate, so that we can differentiate set vs not in
scripts that parse the logs.

Also add some placeholders for warnings that I expect to turn into
errors.

Test: Set BUILD_BROKEN_PHONY_TARGETS to "foo"
Change-Id: I3f7e1e0f85dcef84d6461fc40bdd83dc5bf75ff7