keep logs outside of chroot

keep logs outside of chroot

Every time stage build is restarted, the build logs of the previous run
are lost. This new feature allows the logs to be kept outside the chroot
and thus protected from purging. This new feature forces portage to use
FEATURES="clean-logs" no matter what, but it should have no effect on
users not using the new functionality added by this patch.

This new function is DISABLED per default due to a lack of feedback,
however, it is highly suggested that this is made default for the next
release cycle.