smaccmpilot: accelerometer belongs in sensors, not position
smaccmpilot: add gcs_receive to build
hwf4 usart: change write payloads to const uint8_t*

* were non-const before, which precludes passing strings
smaccmpilot: position gets time tag
smaccmpilot: sensors gets a time tag

* everybody gets a time tag
smaccmpilot: gcs_receive is a wip