xorg-release: ignore some X.org packages

Following Gustavo's removal of two X.org drivers for old hardware
unlikely to be used in embedded contexts, the xorg-release script now
reports those two X.org packages as "to be added": they exist in
X.org, but not in Buildroot.

So, we add a small list, XORG_EXCEPTIONS, in our xorg-release script,
to list the X.org packages we don't want to hear about. Of course,
packages that exist in X.org, and that are not part of this exception
list, and are not packaged in Buildroot are still listed as "to be
added".

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>