config.setcpv: fix bug #522362

Fix config.setcpv to regenerate USE settings in order to account for
package.env USE settings from the previous package instance.

X-Gentoo-Bug: 522362
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=522362
Acked-by: Alexander Berntsen <bernalex@gentoo.org>
Acked-by: Brian Dolbec <dolsen@gentoo.org>