Add OpenOCD support

[jrvanwhy]

OpenSK uses tockloader to deploy the kernel and apps, and currently passes flags that cause tockloader to invoke JLinkExe. We should allow users to use OpenOCD instead of JLinkExe, by making the flags passed to tockloader configurable/changable.

Additionally, because distributions often have an outdated version of OpenOCD that cannot deploy to a nrf52840-DK, OpenSK should allow users to use a custom version of OpenOCD. I've already opened tock/tockloader/pull/52 to enable tockloader to invoke a custom version of OpenOCD.

[jrvanwhy]

The tockloader PR (tock/tockloader#52) has been merged. I opened the following issue to get support for OpenOCD arguments added to tock: tock/tock#1626

[kaczmarczyck]

This would need to be supported in Wasefire with #768, and then we would inherit it automatically. Please open an issue there if you are still interested.