mtlib: update ebuild template

We're killing cros-binary_src_install, so update the template to
install files directly.

BUG=chromium:506366
TEST=None

Change-Id: I95c02cf2382348a3ff553ffa5f55777631d11df8
Reviewed-on: https://chromium-review.googlesource.com/687397
Commit-Ready: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>