Make stdin/stdout/stderr be FileHandles

Rework so that everything is a FileHandle, including
stdin/stdout/stderr.

Provide legacy functionality of calling serial_getc and serial_putc as
an internal "DirectSerial" FileHandle.

Add a JSON option to use UARTSerial instead.

Add hooks for target and application to provide custom FileHandles.

Allow for CRLF conversion to work on any FileHandle that isatty(),
as stdin/stdout or any other FILE. Optimise the conversion so it
doesn't force all write calls to be 1 byte. Limit the conversion
to the stdio layer, so that read() and write() work the same as
the FileHandle methods - this seems less confusing.

Author: kjbracey

platform/mbed_lib.json

@@ -2,7 +2,17 @@
     "name": "platform",
     "config": {
         "stdio-convert-newlines": {
-            "help": "Enable conversion to standard newlines on stdin/stdout",
+            "help": "Enable conversion to standard newlines on stdin/stdout/stderr",
+            "value": false
+        },
+
+        "stdio-convert-tty-newlines": {
+            "help": "Enable conversion to standard newlines on any tty FILE stream",
+            "value": false
+        },
+
+        "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.",
             "value": false
         },