colibri-imx7: use opengldummy for egl and libgles2

Use opengldummy instead mesa for the headers and libs,
which are required only for building Qt and not at runtime.

Change-Id: I82ff661e842375bd68dffb3db790b0309c71d14a
Reviewed-by: Mikko Gronoff <mikko.gronoff@qt.io>
Reviewed-by: Samuli Piippo <samuli.piippo@qt.io>

conf/distro/include/colibri-imx7.conf

@@ -43,4 +43,7 @@ BOOT_SCRIPTS += " \
     u-boot-nand.imx:${MACHINEDIR}/u-boot-nand.imx \
     "
 
+PREFERRED_PROVIDER_virtual/egl = "opengldummy"
+PREFERRED_PROVIDER_virtual/libgles2 = "opengldummy"
+
 DISTRO_FEATURES_remove = "webengine"