xserver-xorg_server: Mention that TinyX is also known as Kdrive

Since several years, the TinyX name has been somewhat deprecated in
favor of Kdrive, so mention the "Kdrive" wording in our configuration
interface.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>