portageq: fix eroot parameter (bug #529200)

The portageq eroot parameter has been broken since commit
c9f6aa9f0151adb3c86706eaef1914cdbdcf2b6d, due to premature instantiation
of portage.settings (before the ROOT variable was set). Premature access
to the portage.settings attribute must be avoided by using other
available means to determine the EPREFIX.

Fixes: c9f6aa9f0151 ("Add cross-prefix support")
X-Gentoo-Bug: 529200
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=529200