You are trying to use a -current kernel config file for a 1.5.2 kernel.
This won't work.

To use 1.5.2 and the (external) I4B distribution, you have to follow the
instructions in NetBSD/INSTALLATION in I4B. Follow them exactly, step by
step.

Or you can use a -current NetBSD system with the GENERIC_ISDN kernel config
file, no I4B needed there.

Martin