Add onboarding key location (specifically for Helium OG definition)

[shawaj]

This is now defined in the settings.toml file in gateway rs:

https://github.com/helium/gateway-rs/blob/900ac54b1d79f79a7d7d840ed2be0ae29e8964c4/config/settings.toml#L11-L12

And was previously in the sys.config:

https://github.com/helium/miner/blob/6739b568e195ef0c66f0b4db3fece0eefde4d873/config/sys.config#L55

Linked with #221 we should allow definition of a separate onboarding key location

[shawaj]

Added in #227