vboot_reference: Decouple from model

The signer has no concept of model and doesn't need to.  From its
perspective, it is simply generating a signature block based on a set of
instructions.

Changing the comments and variable name to reflect this.

BUG=b:68141451
TEST=None
BRANCH=None

Change-Id: Ia2a3e4a5273a4bcd9c5645db2cf0db80af6c28cf
Reviewed-on: https://chromium-review.googlesource.com/733857
Commit-Ready: C Shapiro <shapiroc@google.com>
Tested-by: C Shapiro <shapiroc@google.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-by: Jason Clinton <jclinton@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
1 file changed