binman: Add a bintool implementation for cbfstool

Add a Bintool for this, which is used to run CBFS tests. It supports
the features needed by the tests as well as fetching a binary from
Google Drive. Building it from source is very slow since it is not
separately supported by the coreboot build system and it builds an
entire gcc toolchain before starting.

Signed-off-by: Simon Glass <sjg@chromium.org>