directfb: add help text to Config.in, note broken URL

Adds descriptive help text to Config.in based on content from elinux.org and
Wikipedia. Notes that http://www.directfb.org/ has been broken for more than
five months and suggests using http://elinux.org/DirectFB

 instead.

Signed-off-by: Jonathan Ben-Avraham <yba@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>