Allow running single steps in presubmit.
Allow running single steps in presubmit.

Add ability to run individual presubmit checks using --step.

Bug: 3
Change-Id: Ibcea8e54de2deb2e8159ae1389b86a5c82da0d14

google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed