uart: add init post hook

Signed-off-by: Thomas Ingleby <thomas.c.ingleby@intel.com>

include/mraa_adv_func.h

@@ -60,4 +60,6 @@ typedef struct {
 
     mraa_result_t (*spi_init_pre) (int bus);
     mraa_result_t (*spi_init_post) (mraa_spi_context spi);
+
+    mraa_result_t (*uart_init_post) (mraa_uart_context uart);
 } mraa_adv_func_t;

include/mraa_internal_types.h

@@ -83,3 +83,12 @@ struct _aio {
     int adc_in_fp; /**< File Pointer to raw sysfs */
     int value_bit; /**< 10 bits by default. Can be increased if board */
 };
+
+/**
+ * A structure representing a UART device
+ */
+struct _uart {
+    /*@{*/
+    int index; /**< the uart index, as known to the os. */
+    /*@}*/
+};