vboot_reference: Decouple from model
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 <97eabdef1f1c3aaebe144a81b3e6d8d879701c39@google.com>
Tested-by: C Shapiro <97eabdef1f1c3aaebe144a81b3e6d8d879701c39@google.com>
Reviewed-by: Simon Glass <c00b0378376498bd9cd974c388df8854c0131d27@chromium.org>
Reviewed-by: Aaron Durbin <af0622ab041e8b0f8e5819838dcc93898196e71f@chromium.org>
Reviewed-by: Jason Clinton <903dc7ec7b1e0549bc200a3278469a1b3add1d8d@chromium.org>
Reviewed-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>

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