buildman: Add a flag for reproducible builds

buildman: Add a flag for reproducible builds

This is quite a useful thing to use when building since it avoids small
size changes between commits. Add a -r flag for it.

Also undefine CONFIG_LOCALVERSION_AUTO since this appends the git hash
to the version string, causing every build to be slightly different.

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