This CL refactors and extends the capabilities of roll_boringssl.go,
making it a flexible way to reset the related repositories to JIRI_HEAD,
run jiri update, update the upstream BoringSSL source, regenerate the
GN build files, update the Mozilla certificate bundle, update Zircon's
uboringssl subset, update the garnet manifest, and submit all of these
changes as a topic.

Change-Id: I69fc4c2110b4d960a9b522d5ee0f60c45cdd0c4d