Added newline conversion for stdin

stdin converts the following character sequences:

\n   -> \n
\r\n -> \n
\r   -> \n
\n\r -> \n

For original behaviour, a serial object can be instantiated explicitly

Author: geky

hal/common/retarget.cpp

@@ -88,7 +88,8 @@ FileHandle::~FileHandle() {
 #if DEVICE_SERIAL
 extern int stdio_uart_inited;
 extern serial_t stdio_uart;
-static char stdio_prev;
+static char stdio_in_prev;
+static char stdio_out_prev;
 #endif
 
 static void init_serial() {
@@ -228,11 +229,11 @@ extern "C" int PREFIX(_write)(FILEHANDLE fh, const unsigned char *buffer, unsign
 #if DEVICE_SERIAL
         if (!stdio_uart_inited) init_serial();
         for (unsigned int i = 0; i < length; i++) {
-            if (buffer[i] == '\n' && stdio_prev != '\r') {
+            if (buffer[i] == '\n' && stdio_out_prev != '\r') {
                  serial_putc(&stdio_uart, '\r');
             }
             serial_putc(&stdio_uart, buffer[i]);
-            stdio_prev = buffer[i];
+            stdio_out_prev = buffer[i];
         }
 #endif
         n = length;
@@ -259,7 +260,24 @@ extern "C" int PREFIX(_read)(FILEHANDLE fh, unsigned char *buffer, unsigned int
         // only read a character at a time from stdin
 #if DEVICE_SERIAL
         if (!stdio_uart_inited) init_serial();
-        *buffer = serial_getc(&stdio_uart);
+        while (true) {
+            char c = serial_getc(&stdio_uart);
+            if ((c == '\r' && stdio_in_prev != '\n') ||
+                (c == '\n' && stdio_in_prev != '\r')) {
+                stdio_in_prev = c;
+                *buffer = '\n';
+                break;
+            } else if ((c == '\r' && stdio_in_prev == '\n') ||
+                       (c == '\n' && stdio_in_prev == '\r')) {
+                stdio_in_prev = c;
+                // onto next character
+                continue;
+            } else {
+                stdio_in_prev = c;
+                *buffer = c;
+                break;
+            }
+        }
 #endif
         n = 1;
     } else {