xorg: use BR2_PACKAGE_XORG instead of BR2_PACKAGE_XSERVER_none

The BR2_PACKAGE_XSERVER_none option has gone, so use BR2_PACKAGE_XORG7
instead. It is more logical to test if BR2_PACKAGE_XORG7 is set to
know if an X.org server is available, than testing if
BR2_PACKAGE_XSERVER_none is set to know if an X.org server is *not*
available.

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