Add Serial passthrough function

[RaptorSDS]

it would be nice to have a passthrougt fuction to confige the gps reciver ( some can be switch between rover and base "LC29HDA") or to use direct with a PC , also to see as debug if pins and config are correct

[designer2k2]

Hello @RaptorSDS , look here for that function: #18 this is not a full passthrough but at least you can send commands