package/glmark2: rework selection of flavours

package/glmark2: rework selection of flavours

https://git.buildroot.net/buildroot/commit/?id=cf66a67112cd706d2e71665e45e8f6453fd53963
was not enough to fix building glmark2 because it was possible to
configure buildroot in a way that no flavour would be active:
http://lists.busybox.net/pipermail/buildroot/2019-October/261943.html
http://lists.busybox.net/pipermail/buildroot/2019-October/261944.html

The broken defconfig was fixed
https://git.buildroot.net/buildroot/commit/?id=ca6f9504dd3825fe4fbd0b4a4b3eea4261032d03

but we need to fix the selection of flavours to ensure that at least
on flavour is available:
http://lists.busybox.net/pipermail/buildroot/2019-October/262022.html

Suggested-by: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>