eee701 machine config: fix to use xorg instead of kdrive

eee701 machine config: fix to use xorg instead of kdrive

eFfeM pointed me out that he had a Xvesa binary in his xf46-image,
  for eeepc701 and no Xorg.
So I investigated and found that:
In eee701.conf there were ?= for XSERVER and
  PREFERRED_PROVIDER_virtual/xserver
That was wrong because we required i686-generic.conf which set them
  And so the ?= didn't override the variable.

Note that we can't set theses variables in local.conf anymore because
  they are now overriden by eee701.conf,but In another hand eFfeM pointed me that
  all machine but gta01 and gta02 seem to do the same thing. So I bet that
  not setting theses variables in local.conf is a desirable behaviour.