build_image: Only copy portage files if installed via a non chromeos-dev rdep

The "fixup" only needs to happen if sys-apps/portage was installed under
/usr/local. If sys-apps/portage gets pulled in via a non chromeos-dev
dependency, say chromeos or a board's bsp, we don't need to do anything.

BUG=chromium:311764
TEST=build_image --board=beaglebone dev test
TEST=build_image --board=lumpy dev test
TEST=build_image --board=daisy dev test
TEST=verify make.globals exists under /usr/share/portage/config on the rootfs.

Change-Id: Ic747c7fdc624b6308e5630f296680438da580571
Reviewed-on: https://chromium-review.googlesource.com/191139
Tested-by: Gaurav Shah <gauravsh@chromium.org>
Reviewed-by: David James <davidjames@chromium.org>
Commit-Queue: Gaurav Shah <gauravsh@chromium.org>
1 file changed