expo: Add some scene fields needed for text entry

expo: Add some scene fields needed for text entry

Add the CLI state, a buffer to hold the old value of the text being
edited and a place to save vidconsole entry context. These will be use
by the textline object.

Set an upper limit on the maximum number of characters in a textline
object supported by expo, at least for now.

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