Add openocd option to RevB settings

[sctincman]

With riscv-collab/riscv-openocd#456, the RevB now has OpenOCD support. Add tag to settings.mk to allow use in freedom-e-sdk.

This also requires makefile changes to key off of, but this will need to be merged first I think before the freedom-e-sdk bsp CI tests will pass.

[nategraff-sifive]

Thanks @sctincman! Can you go ahead and submit a PR to freedom-e-sdk with the changes you suggest there to support the HiFive1 RevB optionally using OpenOCD? When we have these cross-repo dependencies we usually use the following workflow:

1. Submit a PR to the child repo
2. Submit a PR to the parent repo, pointing at the child repo branch
3. Review the changes together and show that the tests pass in both PRs
4. Merge the child repo, rebase the parent PR to include child master with the change, and merge the parent repo.

[sifivekevin]

These changes look good on visual review. I cannot test because I lent my HiFive1 RevB target out and haven't received it back yet.

[sctincman]

@nategraff-sifive, certainly! Will do. I was wondering how the e-sdk was handled with its large number of submodules.

Should I also update the submodule target using the child PR commit? I imagine that's what you mean will be rebased ontop of merged child?

Lastly, the makefile change I currently have is built on top of my other PR (sifive/freedom-e-sdk#441), but that seems to be in limbo at the moment. Could I potentially get that reviewed? Or should I make the change on top of what's in master at the moment.