Log Message:
xentools418: Update commented-out SDL2 bl3 line and explain

I recently added a patch to force-disable sdl2-config.  Greg Woods
pointed out that if one adds SDL2 bl3 (vs SDL that was commented-out
previously), the package builds.  This commit just updates the
commented-out line and explains this.  (I have no reason to think
including SDL2 is a good idea.)