video/console: Implement reverse video ANSI sequence for DM_VIDEO

The video console for DM_VIDEO compliant drivers only understands a very
small number of ANSI sequences. First and foremost it misses the "reverse
video" command, which is used by our own bootmenu command to highlight
the selected entry.

To avoid forcing people to use their imagination when using the
bootmenu, let's just implement the rather simple reverse effect. We need
to store the background colour index for that, so that we can
recalculate both the foreground and background colour pixel values.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
[agust: merged BG color escape seq change to fix "ut dm video_ansi" test]
Signed-off-by: Anatolij Gustschin <agust@denx.de>