package/openjdk: only support a single variant at a time

package/openjdk: only support a single variant at a time

The Zero variant won't build with the server or client also selected at the same
time, and expressing this in the Config.in would be too complicated to do.
Even so, selecting multiple variants doesn't seem to be that important in the
context of Buildroot.

This patch removes the ability to select multiple variants in favor
of just selecting one. The default is server as that is what all of the major
distributions currently use as the default as well.

Fixes:
http://autobuild.buildroot.net/results/a45cfa9b3602fd05f6adbf070a1bad6510975c36

Signed-off-by: Adam Duskett <Aduskett@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>