Store ssh test keys to build output root

BUG=brillo:1200
TEST=Ran build_packages with the USE flag 'generated_ssh_key' set and verified
the right thing was done. Also ran a trybot with rambi-pre-cq and
x86-generic-paladin, which didn't have the flag set, to make sure default
workflow was not affected.
CQ-DEPEND=CL:292312

Change-Id: I0d8790a92ff21c8516878717da34f17ab7b062d7
Reviewed-on: https://chromium-review.googlesource.com/286944
Reviewed-by: Richard Barnette <jrbarnette@chromium.org>
Reviewed-by: Prathmesh Prabhu <pprabhu@chromium.org>
Tested-by: Daniel Wang <wonderfly@google.com>
Commit-Queue: Daniel Wang <wonderfly@google.com>
3 files changed