build_sdk_board: move crossdev overlay init here

Set this up when creating the SDK rather than when unpacking it.

BUG=b:191307774
TEST=CQ passes

Change-Id: Id717ca7ee4e3952905b0ec9e8dba51f6f38f9892
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosutils/+/4851348
Reviewed-by: Jack Rosenthal <jrosenth@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>