Add support for sharing DEBUG_TRACE port with an MSP port

- Added MSPv2 code MSP2_COMMON_DEBUG_OUTPUT, which sends a debug
message to the MSP client. It must be ready to accept MSP responses
without sending a request first.
- Moved MSP initialization a bit early in the boot process, so we
can print debug output during HW initialization.
- Added a couple of functions in msp_serial.c to locate an MSP port
from a serial port and to push a message to a given MSP port.
- Updated isSerialConfigValid() to allow sharing MSP with DEBUG_TRACE.

src/main/fc/fc_init.c

@@ -255,6 +255,10 @@ void init(void)
     serialInit(feature(FEATURE_SOFTSERIAL), SERIAL_PORT_NONE);
 #endif
 
+    // Initialize MSP here so the DEBUG_TRACE can share a port with MSP
+    mspFcInit();
+    mspSerialInit();
+
 #if defined(USE_DEBUG_TRACE)
     // Debug trace uses serial output, so we only can init it after serial port is ready
     // From this point on we can use DEBUG_TRACE() to produce real-time debugging information
@@ -550,9 +554,6 @@ void init(void)
 
     imuInit();
 
-    mspFcInit();
-    mspSerialInit();
-
 #ifdef USE_CLI
     cliInit(serialConfig());
 #endif