Add IBD build configs for BCHN network

Summary:
See title.

Depends on D8289

Test Plan:

./build-configurations.py build-ibd-bchn
./build-configurations.py build-ibd-no-assumevalid-checkpoint-bchn

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8299