package/gstreamer1: gl enabled when either GL or GLES2 is present

When building gst1-plugins-base with GL support on the rpi3 (which
supports only GLES2, not full desktop GL), GStreamer was being asked to
disable its GL support completely. This isn't correct since it can be
used with either GL or GLES2. There is already a Config.in symbol to
indicate that either of them is enabled:
BR2_PACKAGE_GST1_PLUGINS_BASE_LIB_OPENGL_HAS_API. So use that to
determine if gl should be enabled or disabled.

While we're at it, also move the GL/GLES handling under the same API
condition. This makes it clearer that they belong together.

Signed-off-by: Charlie Turner <cturner@igalia.com>
Reviewed-by: Adrian Perez de Castro <aperez@igalia.com>
[Arnout: refactor using
BR2_PACKAGE_GST1_PLUGINS_BASE_LIB_OPENGL_HAS_API]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>