sys-block/open-iscsi: prevent endless changes of /etc/iscsi/initiatorname.iscsi.

sys-block/open-iscsi: prevent endless changes of /etc/iscsi/initiatorname.iscsi.

Upstream now generates /etc/iscsi/initiatorname.iscsi during the install
phase, but it is NOT deterministic. The content of the file should be
generated ONCE and kept over upgrades & reinstalls.

The upstream Makefile assumes that DESTDIR is the rootfs, and generates
/etc/iscsi/initiatorname.iscsi anytime it does not exist.

Build /etc/iscsi/initiatorname.iscsi ourselves, as we used to do in
older versions.

Fixes: https://bugs.gentoo.org/show_bug.cgi?id=601514
Gentoo-Bug: 601514
Package-Manager: portage-2.3.6
Signed-off-by: Robin H. Johnson <robbat2@gentoo.org>