video: Support showing a cursor

video: Support showing a cursor

Add rudimentary support for displaying a cursor on a vidconsole. This
helps the user to see where text is being entered.

The implementation so far is very simple: the cursor is just a vertical
bar of fixed width and cannot be erased. To erase the cursor, the text
must be redrawn over it.

This is good enough for expo but will need enhancement to be useful for
the command-line console. For example, it could save and restore the
area behind the cursor.

For now, enable this only for expo, to reduce code size.

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