meta-google: gbmc-ncsi-config: Only recognize impersonation

This is the only case where we need to add the rule, any addresses that
are BMC specific have a different workflow for assigning the firewall
rules.

Change-Id: If15c0cddb23044f774b55b8585446f923c76d7e8
Signed-off-by: William A. Kennington III <wak@google.com>