Make some adjustments so that it's possible to install binary

Make some adjustments so that it's possible to install binary
packages without having a portage tree:

* Make portdbapi.aux_get() return early by raising a KeyError
  if it detects that there is no portage tree.

* Move the ARCH and USERLAND sanity check to the last moment
  in doebuild() and only require these variables if an
  existing environment (such as environment.bz2) is
  unavailable.

* Convert messages about missing portage tree to debug level
  warnings since quite often it's possible to function without
  a portage tree now.

* Make binarytree.populate() stop relying on having a categories
  list since it may not be available if there is no portage tree.

(trunk r8972:8976)

svn path=/main/branches/2.1.2/; revision=8977