Add option to disable default UART console

New `target.console-uart` option added to indicate whether a target has
a console UART on STDIO_UART_TX/RX/RTS/CTS pins. (The existing option
`target.console-uart-flow-control` indicates whether RTS and or CTS is
available in addition to TX and RX).

The option defaults to true, and is currently true on all platforms. It
only applies if DEVICE_SERIAL is true, so no need to go through and mark
it false for non-SERIAL platforms.

An application can turn off target.console-uart to save ROM/power/etc if
they don't want to use the serial console.  If this is turned off, the
console won't be activated for stdin/stdout, but the application is
still free to open `UARTSerial(STDIO_UART_TX, STDIO_UART_RX)`
themselves.

platform/mbed_lib.json

@@ -12,12 +12,12 @@
         },
 
         "stdio-buffered-serial": {
-            "help": "Use UARTSerial driver to obtain buffered serial I/O on stdin/stdout/stderr. If false, unbuffered serial_getc and serial_putc are used directly.",
+            "help": "(Applies if target.console-uart is true.) Use UARTSerial driver to obtain buffered serial I/O on stdin/stdout/stderr. If false, unbuffered serial_getc and serial_putc are used directly.",
             "value": false
         },
 
         "stdio-baud-rate": {
-            "help": "Baud rate for stdio",
+            "help": "(Applies if target.console-uart is true.) Baud rate for stdio",
             "value": 9600
         },
 

platform/mbed_retarget.cpp

@@ -261,7 +261,7 @@ MBED_WEAK FileHandle *mbed::mbed_override_console(int fd)
 
 static FileHandle *default_console()
 {
-#if DEVICE_SERIAL
+#if MBED_CONF_TARGET_CONSOLE_UART && DEVICE_SERIAL
 #  if MBED_CONF_PLATFORM_STDIO_BUFFERED_SERIAL
     static UARTSerial console(STDIO_UART_TX, STDIO_UART_RX, MBED_CONF_PLATFORM_STDIO_BAUD_RATE);
 #   if   CONSOLE_FLOWCONTROL == CONSOLE_FLOWCONTROL_RTS
@@ -274,7 +274,7 @@ static FileHandle *default_console()
 #  else
     static DirectSerial console(STDIO_UART_TX, STDIO_UART_RX, MBED_CONF_PLATFORM_STDIO_BAUD_RATE);
 #  endif
-#else // DEVICE_SERIAL
+#else // MBED_CONF_TARGET_CONSOLE_UART && DEVICE_SERIAL
     static Sink console;
 #endif
     return &console;

targets/targets.json

@@ -15,6 +15,10 @@
         "bootloader_supported": false,
         "static_memory_defines": true,
         "config": {
+            "console-uart": {
+                "help": "Target has UART console on pins STDIO_UART_TX, STDIO_UART_RX. Value is only significant if target has SERIAL device.",
+                "value": true
+            },
             "console-uart-flow-control": {
                 "help": "Console hardware flow control. Options: null, RTS, CTS, RTSCTS.",
                 "value": null