Add ThreadDebug serial port string

To use, add this snippet on the top of your project

```
 #include <ThreadDebug.h>

 USBSerial SerialUSB1(false, "DebugPort");
 UsbDebugCommInterface  debugComm(&SerialUSB1);
 ThreadDebug            threadDebug(&debugComm, DEBUG_BREAK_IN_SETUP);
```

Author: facchinm

boards.txt

@@ -47,6 +47,13 @@ envie_m7.bootloader.programmer=-f interface/stlink.cfg
 envie_m7.bootloader.extra_action.preflash=stm32h7x option_write 0 0x01c 0xb86aaf0
 envie_m7.bootloader.file=PORTENTA_H7/portentah7_bootloader_mbed_hs_v2.elf
 
+envie_m7_thread_debug.name=Arduino Portenta H7 (ThreadDebug)
+envie_m7_thread_debug.vid.0=0x2341
+envie_m7_thread_debug.pid.0=0x025B
+envie_m7_thread_debug.descriptor.0=DebugPort
+envie_m7_thread_debug.hide=true
+envie_m7_thread_debug.build.board=PORTENTA_H7_M7
+
 ##############################################################
 
 envie_m4.name=Arduino Portenta H7 (M4 core)