generic_stage_target: default {PORT,PKG,DIST}DIR

generic_stage_target: default {PORT,PKG,DIST}DIR

This provides forward compatiblity so that stage builds do not break
in the future when portage changes the default PORTDIR, PKGDIR, and
DISTDIR locations (bug #378603).

Also, it provides backward compatibility for various tools that require
PORTDIR to be defined in make.conf, such as the following:

	gentoo-bashcomp - bug #478444
	euse - bug #474574
	euses and ufed - bug #478318

X-Gentoo-Bug: 479062
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=479062
Reviewed-by: Matt Turner <mattst88@gmail.com>