Prevent interface from coming up on boot

[kimble]

Same question as in this thread:
Re: Prevent interface from coming up on boot

...but for different reasons: My ISP allows DSL customers to connect from the wider internet over L2TP using the same login details. This allows you to use a different ISP as a backup, while maintaining the same routed IP addresses.
The problem is that as soon as the L2TP tunnel comes up, the ISP routes all packets to that one preferentially, even if the DSL link is still active.

What I want is for pfsense to start up with the L2TP interface down, so the DSL can be used normally. I'm fine with manually bringing it up during an outage (indeed, this is preferable, as I'm using a PAYG cellular connection as the backup).

Disabling the interface causes havoc with the gateway configuration that I'd rather avoid.