Make jerryx_port_handler_print_char truly a port function (#2789)

In the previous approach `jerryx_port_handler_print_char` was implemented
in by the jerry-default-port. This implementation however required the
jerry-ext handler header file which created a requirement in the
jerry-default-port for the jerry-ext headers.

JerryScript-DCO-1.0-Signed-off-by: Peter Gal pgal.u-szeged@partner.samsung.com

jerry-core/include/jerryscript-port.h

@@ -179,6 +179,18 @@ struct jerry_context_t *jerry_port_get_current_context (void);
  */
 void jerry_port_sleep (uint32_t sleep_time);
 
+/**
+ * Print a single character.
+ *
+ * Note:
+ *      This port function is here so the jerry-ext components would have
+ *      a common way to print out information.
+ *      If possible do not use from the jerry-core.
+ *
+ * @param c the character to print.
+ */
+void jerry_port_print_char (char c);
+
 /**
  * @}
  */