FPC experiments: got so far TX_only, usart=115200 + usb=460800, see full commit msg

* Add \r\n to sent strings
* remove usart_init() from UsbPacketReceive cmd, it's already init in main.
* Add PLATFORM PM3RDV4FPC to ease dev
* TX: US_TCR is len of data to send, not len of buffer
* Use only one PDC bank as we're using it in sync
* Busy loop to wait for end of TX as we'using it in sync
* Change usart speed to 115200
* Don't downgrade USB speed, keep 460800
* Attempt to detect received data, fail so far

common/usart.h

@@ -9,4 +9,5 @@ void usart_close(void);
 
 int16_t usart_readbuffer(uint8_t *data, size_t len);
 int16_t usart_writebuffer(uint8_t *data, size_t len);
+bool usart_dataavailable(void);
 #endif