cros payload: --signatures flag shows the metadata signatures

The existing --signatures flag now dumps the metadata signatures
embedded in the payload.

BUG=None
TEST=Added unittests; manually check a signed payload.

Change-Id: I4e2eb5cafeabbafd40af31716840d405a79b1023
Reviewed-on: https://chromium-review.googlesource.com/310654
Commit-Ready: Alex Deymo <[email protected]>
Tested-by: Alex Deymo <[email protected]>
Reviewed-by: Sen Jiang <[email protected]>
2 files changed