packagegroup-core-x11-xserver: Add XSERVERCODECS variable

packagegroup-core-x11-xserver: Add XSERVERCODECS variable

At present its hard for the system to provide extra x11 hardware codecs.
We could lump them in with the XSERVER variable but this is suboptimal as
in some use cases the user might not want to include them in some images.

With this patch we keep our options open about separating usage of the two
in the future.

(From OE-Core rev: 285c7577813cf75a3b554f8a5d6d7ba002127ef4)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>