Add cross-prefix support

This patch series adds support for using a portage installed in one prefix
to build packages with a different prefix.

The current portage has a single EPREFIX variable specifying both the prefix
of the portage installation, and the prefix of the packages portage is
building. This patch series splits it into two parts: the
portage.const.EPREFIX variable specifying the prefix of the portage
installation, used for constructing the PATH as well as the paths to files
belonging to a portage installation itself rather than a target root; and the
 EPREFIX setting in config instances, specifying the prefix of the
to-be-built packages and being used for almost everything else.

The EPREFIX config setting defaults to const.EPREFIX, but can be overridden
by the EPREFIX environment variable, as well as the emerge --prefix option.
This allows one to install systems with different prefixes using
`EPREFIX=/foo emerge @system`, though some unrelated changes need to happen
elsewhere first in order to make that a reality.

Ruud Koolen (3):
  Distinguish between portage prefix and package prefix
  Based GLOBAL_CONFIG_PATH and DEPCACHE_PATH on portage prefix
  Pick up EPREFIX environment variable