extension: Refactor to allow non-command usage

The current extension code is designed to be used from commands. We want
to add a boot driver which uses it. To help with this, split the code into
the command processing and a function which actually does the scan.

Really the extension code should be in common/ or use driver model, but
this is a start.

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