[c-nsp] RPR between 720-3B and 720-3BXL

[Kevin Graham]

I could have sworn this had been covered on the list before, but I can't
find it in archives.

We need to get a switch w/ Sup720-3B's upgraded to a Sup720-3BXL's. Though
I'm sure its not supported, does anyone know if a (same generation but
larger) PFC can come up as a standby? SSO seems too much to hope for, so
presumably this would just be RPR.

The only interest is in running this way for ~10m to sync up configs, etc to
the first BXL before switching over to it and swapping out the other sup.