This is to make cable issues or charge-only usb cables easily visible for the user.

Use a slow blink (not aggressively fast) because this could also be a permanent and expected state (Glasgow with flashed gateware connected to a power supply).

Implementation: check in firmware if an address is assigned.

Idea from @whitequark on irc: https://freenode.irclog.whitequark.org/glasgow/2020-12-20#28644811;