FPC: got RX working, got client over usart somehow working..., see detailed commit msg

* using WITH_FPC:
  * activate basic usart
  * no double buffer for now, no interrupt
  * usart_dataavailable/usart_readbuffer/usart_writebuffer, to demo it:
    * pm3 client over USB, minicom over usart
    * analyse a d 414243
* using WITH_FPC_HOST:
  * it implies WITH_FPC as it's based on it
  * control pm3 with client over usart
  * EXPERIMENTAL! still some frame desync issues
  * you can connect both from usart & USB with two pm3 clients
    * actually you *have* to connect USB for the moment because
      it's used to send debug messages about buggy usart... See Dbprintf_usb below
  * "sessions": msgs are directed to the latest client to have sent a cmd
  * Dbprintf_usb macro to send msgs to USB client to help debugging usart...
  * We now have an option to run client at different speed as usart is 115200:
    client/proxmark3 /dev/ttyUSB0 -b 115200
  * Consequently, argc,argv handling is a bit revamped, it was so messy...
  * USB and flashing are still at 460800, don't try flashing over usart yet ^^

include/usb_cmd.h

@@ -34,6 +34,12 @@ typedef struct {
         uint32_t asDwords[USB_CMD_DATA_SIZE / 4];
     } d;
 } PACKED UsbCommand;
+
+#ifdef WITH_FPC_HOST
+    // "Session" flag, to tell via which interface next msgs should be sent: USB or FPC USART
+    extern bool reply_via_fpc;
+#endif
+
 // A struct used to send sample-configs over USB
 typedef struct {
     uint8_t decimation;