releasetools: Fix parameters for vboot_signer

With the change in vboot_signer (CL:744257), an additional parameter
is required to vboot_signer script. This change adds the required
parameter to releasetools as well.

BUG=23076037

Change-Id: Ice5329578b3a9bc459e278a9d404a1981b35ca88
Signed-off-by: Furquan Shaikh <furquan@google.com>