design-2.1: Update OS Flavours section

This reflects a discussion we had, according to which the full
"parameters" implementation is too heavy weight for 2.1, and we should
have a partial version for now, and decide again later.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>