Fix mbed_board.c compiler warning

Fix the following compiler warning.

Compile [ 63.7%]: mbed_board.c
[Warning] mbed_board.c@99,36: comparison between signed and unsigned integer expressions [-Wsign-compare]

Signed-off-by: Tony Wu <[email protected]>

Author: tung7970

platform/mbed_board.c

@@ -88,15 +88,15 @@ void mbed_error_vfprintf(const char * format, va_list arg) {
             serial_init(&stdio_uart, STDIO_UART_TX, STDIO_UART_RX);
         }
 #if MBED_CONF_PLATFORM_STDIO_CONVERT_NEWLINES
-        for (unsigned int i = 0; i < size; i++) {
+        for (int i = 0; i < size; i++) {
             if (buffer[i] == '\n' && stdio_out_prev != '\r') {
                  serial_putc(&stdio_uart, '\r');
             }
             serial_putc(&stdio_uart, buffer[i]);
             stdio_out_prev = buffer[i];
         }
 #else
-        for (unsigned int i = 0; i < size; i++) {
+        for (int i = 0; i < size; i++) {
             serial_putc(&stdio_uart, buffer[i]);
         }
 #endif