package/xserver_xorg-server: make the version a choice

Only a single entry for now, but we'll add one more in an upcoming
patch, to support old binary-only drivers, like the NVidia Tegra2/3
drivers.

[Thomas:
 - rename option from BR2_PACKAGE_XSERVER_XORG_SERVER_V_1_17_1 to
   BR2_PACKAGE_XSERVER_XORG_SERVER_V_1_17, since we don't want to
   rename the option at each minor release of the X.org server, and
   X.org servers with the same major version will not have a different
   ABI.]

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>