eclass: update readme.gentoo utils for EAPI=7

BUG=chromium:926296
TEST=CQ passes

Change-Id: I19b574b663e6d8684d752e88590377ea384c1200
Reviewed-on: https://chromium-review.googlesource.com/1767816
Tested-by: Mike Frysinger <vapier@chromium.org>
Commit-Ready: Mike Frysinger <vapier@chromium.org>
Legacy-Commit-Queue: Commit Bot <commit-bot@chromium.org>
Reviewed-by: Chris McDonald <cjmcdonald@chromium.org>
2 files changed