video/console: Implement ANSI clear line command

There is a standard ANSI terminal escape sequence to clear a whole line
of text. So far the DM_VIDEO console was missing this code.

Detect the sequence and use vidconsole_set_row with the background
colour to fix this omission.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Simon Glass <sjg@chromium.org>