Added an environment for pico using HW serial

I wasn't sure how to compile with or without
HW serial support, so I duplicated the "pico"
environment and added a -D HW_SERIAL

src/config.h

@@ -3,9 +3,11 @@
     #define BOARD_AUTO
     //#define BOARD_SANGABOARDv5
 
-    //without this, default to HW serial where available
+    //if HW_SERIAL is not defined, we'll use USB serial communication
     //the details are board specific and configured in boards.h
+    #ifndef HW_SERIAL
     #define USB_SERIAL
+    #endif
 
     #include <boards.h> //defines wiring