Make BootAddrReg optional

[jerryz123]

Previously, the BootAddrReg device is always instantiated on all designs.
This change makes that device optional, so it can be removed from designs with no tiles.

This also adds the config flag for CustomBootPin to AbstractConfig, so its clearer what the set of "default devices" includes.

Related PRs / Issues:

Type of change:

• Bug fix
• New feature
• Other enhancement

Impact:

• RTL change
• Software change (RISC-V software)
• Build system change
• Other

Contributor Checklist:

• Did you set main as the base branch?
• Is this PR's title suitable for inclusion in the changelog and have you added a changelog:<topic> label?
• Did you state the type-of-change/impact?
• Did you delete any extraneous prints/debugging code?
• Did you mark the PR with a changelog: label?
• (If applicable) Did you update the conda .conda-lock.yml file if you updated the conda requirements file?
• (If applicable) Did you add documentation for the feature?
• (If applicable) Did you add a test demonstrating the PR?

• (If applicable) Did you mark the PR as Please Backport?

[T-K-233]

Looks good!