keygeneration: add helper for generating update payload keys
keygeneration: add helper for generating update payload keys

We don't use this anywhere as we've only ever generated one key so
far.  But we never wrote this down, so this is more documentation.

BUG=None
TEST=ran the code manually
BRANCH=None

Change-Id: Ia9a318c686b1ad7ab1de31899b49ce73a4d5ad9f
Signed-off-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/1947554
Reviewed-by: George Engelbrecht <c828e237e346dc3df684bcfc0b60e48b180f6a3d@google.com>

coreboot/vboot,coreboot/vboot,coreboot/vboot,coreboot/vboot,coreboot/vboot