Make cfgupgrade idempotent

Make cfgupgrade idempotent

One of the unit tests checks whether cfgupgrade's downgrade option is
idempotent. It is, but a version number check made it impossible to
actually downgrade multiple times.

With this change, both the current-version version number and the
target-version version number are accepted by the downgrade tool.

Signed-off-by: Michele Tartara <mtartara@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>