build 32-bit images on 64-bit systems

Document how to build 32-bit images on 64-bit systems, and enhance
the model cache cache to take architecture into account so that a
single model cache file is not shared between 32-bit and 64-bit
builds if a flavorless system model is used.

Signed-off-by: Michael K Johnson <johnsonm@foresightlinux.org>

Fixes #1