meta-yocto/conf/machine/beagleboard.com hardcodes the virtual/xserver provider, i.e., it includes the following: PREFERRED_PROVIDER_virtual/xserver = "xserver-xorg-lite" I don't think machine conf is the correct place for selecting what is essentially a distro feature, but at least this should use the '?=' operator; as is to select a different provider one must provide a complete beagleboard.conf file.
Merged in master: http://git.yoctoproject.org/cgit/cgit.cgi/poky/commit/?id=0f21f39fad3717daef733fac3e9eb81976a3c866