dev-install: fix target of local make.profile

We need to point to the make.profile, not the entire portage config.

BUG=chromium:194889
TEST=precq passes & created profile works w/emerge

Change-Id: I3af3dbbaceecf8c17747d1f536d58783baea7970
(cherry picked from commit a02cc337a6cc9d11f9c4d1cc34e17e637a57f48f)
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/1518289
Reviewed-by: Chris McDonald <cjmcdonald@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>