Per-Node Serial Baud Rate

[dgoodlad]

Is your feature request related to a problem? Please describe.
Rockchip-based devices are typically shipped with a debug UART that communicates at 1500000 baud. bmcd naively configures all four serial ports to 115200 baud, since that's the default for Raspberry Pi devices (and others, I'm sure). The current uart API is, therefore, unusable for these Rockchip devices.

Describe the solution you'd like
Ideally, bmcd could auto-detect the appropriate baud rate for a given node's device. This may be through a mapping of known device types to rates, or autobaud detection. This would probably need to be overridable by the user as well, in case of unknown or broken detection.

So, for a given node, the baud rate could be configured by falling through values from this list, in order:

1. User-specified baud rate for single node
2. User-specified baud rate override for all nodes (maybe?)
3. Known baud rate for a known device attached to the node
4. Auto baud rate detection
5. Fallback, default rate of 115200

Maybe an initial implementation could be as simple as items 1 and 5?

Describe alternatives you've considered
Look, you can always ssh in and use picocom to set the baud rate, but that's not ideal.

Additional context
A device that exhibits this behaviour is the Orange Pi CM4.