mbed_retarget: Make filehandles optional to reduce code size

The retarget code allocates an array of FileHandle* for console and file
handling (filehandles). A tiny target only needs a console (putc/getc).
There is no need for file handling.
The POSIX layer and the array of FileHandle* is not required for small
targets that only need a console ; this code should be optionally compiled
out if the configuration parameter `platform.stdio-console-only` is
selected instead of `platform-stdio-buffered-serial`.

Author: hugueskamba

platform/mbed_lib.json

@@ -16,6 +16,11 @@
             "value": false
         },
 
+        "stdio-minimal-console-only": {
+            "help": "(Applies if target.console-uart is true.) Enable this instead of `stdio-buffered-serial` if filehandles are not required to access the serial interface as console to only to print.",
+            "value": false
+        },
+
         "stdio-baud-rate": {
             "help": "(Applies if target.console-uart is true.) Baud rate for stdio",
             "value": 9600

platform/mbed_retarget.h

@@ -586,6 +586,10 @@ extern "C" {
     long telldir(DIR *);
     void seekdir(DIR *, long);
     int mkdir(const char *name, mode_t n);
+#if MBED_CONF_PLATFORM_STDIO_MINIMAL_CONSOLE_ONLY
+    ssize_t minimal_console_putc(const void *buffer, size_t length);
+    ssize_t minimal_console_getc(void *buffer, size_t length);
+#endif // MBED_CONF_PLATFORM_STDIO_MINIMAL_CONSOLE_ONLY
 #if __cplusplus
 }; // extern "C"