Add uart_rxb

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

sw/uart.s

@@ -2,8 +2,8 @@
 
 .importzp sp, sreg
 
-.export _uart_txb
-.export _uart_txb_block
+.export _uart_txb, _uart_txb_block
+.export _uart_rxb
 .export _uart_status
 
 .autoimport	on
@@ -23,6 +23,11 @@ _uart_txb_block:
         bne @1
         rts
 
+_uart_rxb:
+        lda UART_RXB        ; Read value
+        ldx #$00
+        rts
+
 _uart_status:
         lda UART_STATUS
         ldx #$00