xorg: remove the XSERVER variable

The XSERVER variable used to be defined by package/Makefile.in because
the X server package name was different depending on the type of X
server that was choosen. Nowadays, the name of the package is always
xserver_xorg-server, so there's no point in having this XSERVER
intermediate variable.

This patch makes all packages use xserver_xorg-server directly as a
dependency, and removes the XSERVER variable from package/Makefile.in.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>