expo: Add basic implementation

An expo is a way of presenting and collecting information from the
user. It consists of a collection of 'scenes' of which only one is
presented at a time. An expo is typically used to show a boot menu
and allow settings to be changed.

One created, the same expo can be automatically presented in graphical
form using a vidconsole, or in text form on a serial console.

Add an initial implementation of the expo itself. Supports for scenes
and objects is provided later.

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