Allow re-targeting of naclports install.

This allows users to define NACLPORTS_PREFIX in order to
have naclports build into a custom location rather than
directly into the toolchain dir.

This change renames:
 NACL_SDK_USR
 NACL_SDK_USR_INCLUDE
 NACL_SDK_USR_LIB
to:
 NACLPORTS_PREFIX
 NACLPORTS_INCLUDE
 NACLPORTS_LIBDIR

BUG=None

Review URL: https://codereview.chromium.org/11885025

git-svn-id: http://naclports.googlecode.com/svn/trunk/src@657 7dad1e8b-422e-d2af-fbf5-8013b78bd812