event: Add a script to decode the event-spy list

For debugging and dicoverability it is useful to be able to see a list of
each event spy in a U-Boot ELF file. Add a script which shows this, along
with the event type and the source location. This makes events a little
easier to use than weak functions, for example.

Add a basic sandbox test as well. We could provide a test for other
boards, but for now, few use events.

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