Add uart_rxb

Once you receive a uart interrupt you can call this function to get the
received character.

sw/uart.h

@@ -6,6 +6,8 @@
 void uart_txb(uint8_t val);
 void uart_txb_block(uint8_t val);
 
+uint8_t uart_rxb();
+
 uint8_t uart_status();
 
 #endif