Current implementation of expandVarref has the following undesired characteristics:

- ref = '${%s}' % variable
  This line discriminates this type of variable reference ${@d.getVar('LAYERDIR')}"
  which causes them to always resolve to None

- referrervalue = self.getVar(key, False)
  if referrervalue and isinstance(referrervalue, str) and ref in referrervalue:
      self.setVar(key, referrervalue.replace(ref, value))
  This block (the serVar() call in particular) has the effect,
  that variables which reference LAYERDIR lose their append/prepend/remove data.
  During the call to expandVarref('LAYERDIR'), whichever append/prepend/remove
  apply at the moment are used by getVar(), and then they are all dropped by serVar(),
  including the ones whose OVERRIDES would come from layers parsed later