cc3200: Re-add support for UART REPL (MICROPY_STDIO_UART setting).

UART REPL support was lost in os.dupterm() refactorings, etc. As
os.dupterm() is there, implement UART REPL support at the high level -
if MICROPY_STDIO_UART is set, make default boot.py contain os.dupterm()
call for a UART. This means that changing MICROPY_STDIO_UART value will
also require erasing flash on a module to force boot.py re-creation.

cc3200/mptask.c

@@ -98,7 +98,12 @@ static FATFS *sflash_fatfs;
 
 static const char fresh_main_py[] = "# main.py -- put your code here!\r\n";
 static const char fresh_boot_py[] = "# boot.py -- run on boot-up\r\n"
-                                    "# can run arbitrary Python, but best to keep it minimal\r\n";
+                                    "# can run arbitrary Python, but best to keep it minimal\r\n"
+                                    #if MICROPY_STDIO_UART
+                                    "import os, machine\r\n"
+                                    "os.dupterm(machine.UART(0, " MP_STRINGIFY(MICROPY_STDIO_UART_BAUD) "))\r\n"
+                                    #endif
+                                    ;
 
 /******************************************************************************
  DECLARE PUBLIC FUNCTIONS

py/misc.h

@@ -46,6 +46,10 @@ typedef unsigned int uint;
 #define MAX(x, y) ((x) > (y) ? (x) : (y))
 #endif
 
+// Classical double-indirection stringification of preprocessor macro's value
+#define _MP_STRINGIFY(x) #x
+#define MP_STRINGIFY(x) _MP_STRINGIFY(x)
+
 /** memory allocation ******************************************/
 
 // TODO make a lazy m_renew that can increase by a smaller amount than requested (but by at least 1 more element)