image_signing: support mounted rootfs in more scripts

Update these scripts to accept a rootfs dir as input so we don't have
to loopback+mount+umount with every invocation.  This speeds up the
overall runs.

BUG=None
TEST=scripts still work against image & rootfs dirs
BRANCH=None

Change-Id: I23050faebefd0a19e8ad44cdb76d7cc49c28e570
Signed-off-by: Mike Frysinger <vapier@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/2923827
Reviewed-by: George Engelbrecht <engeg@google.com>