bootstd: Add the concept of a bootdev hunter

bootstd: Add the concept of a bootdev hunter

Some bootdevs must be enumerated before they appear. For example, USB
bootdevs are not visible until USB is enumerated.

With standard boot this needs to happen automatically, since we only
want to enumerate a bus if it is needed.

Add a way to define bootdev 'hunters' which can be used to hunt for
bootdevs of a given type. Track which ones have been used and add a
command to list them.

Include a clang work-around which seems to be needed.

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