build_sdk_board: setup portage cache dirs

Create these stub dirs when creating the SDK instead of everytime
we unpack the SDK.

BUG=b:191307774
TEST=CQ passes

Change-Id: I6e512ac49c623f98d9a0c0d0416b0e21cef3ca89
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosutils/+/4861906
Tested-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Alex Klein <saklein@chromium.org>
1 file changed