`serialConfig` baud rate default value clarification

[nebrius]

According to the spec, the baud property is optional, however we don't specify in the spec what the default value is when baud is not included.

Given there's no "this board prefers this rate" notion, I think we should explicitly specify a baud rate. I feel like we should set it to 9600, but what do you all think?

[rwaldron]

Works for me!