keygeneration: add --no-pk option for UEFI key generation

In case PK has been generated in HSM, no need to generate them in
software.

BUG=b:62189155
TEST=See CL:*630434.
BRANCH=none

Change-Id: I2180b340e992b678e46920a1142d3b7101c8158f
Reviewed-on: https://chromium-review.googlesource.com/1071242
Commit-Ready: Edward Jee <edjee@google.com>
Tested-by: Edward Jee <edjee@google.com>
Reviewed-by: Mike Frysinger <vapier@chromium.org>