[Feature] Group hub system functions under hub.system

[laurensvalk]

Originally posted by @laurensvalk in #272 (comment)

It would be nice to group system functions, perhaps like this:

hub.system.set_stop_button()
hub.system.reset()
hub.system.reset_reason()

There may also be a few read only attributes like bluetooth_address. If we have a lot of them maybe these can be put in a dictionary like this:

hub.system.info()

Other system related things:

• Used / free flash space (Prime Hub / Inventor Hub)
• uptime ?
• Hardware version
• Hub name (We have an open issue for a use case somewhere)