Add RF to config templates for "Merge" layers

[vloncar]

Description

Recent changes to nnet_merge_stream.h make it rely on RF, which is not set in the config template. This adds the missing functionality. I've added it across all backends, even though some don't rely on the feature (yet). Closes #1088

Type of change

• Bug fix (non-breaking change that fixes an issue)

Tests

This would show up if we synthesized the projects from test_merge (that use vivado + io_stream)

Checklist

• I have read the guidelines for contributing.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.
• I have installed and run pre-commit on the files I edited or added.
• I have added tests that prove my fix is effective or that my feature works.