Add basic UART driver

This didn't have a chance to get tested so I hope it works.

There is not fancy stuff here, just write to the register and wait till
it is done.

sw/io.inc65

@@ -1 +1,6 @@
 SEVEN_SEG   = $7ff0
+
+UART        = $7ff4
+UART_TXB    = UART
+UART_RXB    = UART
+UART_STATUS = UART + 1

sw/uart.h

@@ -0,0 +1,11 @@
+#ifndef _UART_H
+#define _UART_H
+
+#include <stdint.h>
+
+void uart_txb(uint8_t val);
+void uart_txb_block(uint8_t val);
+
+uint8_t uart_status();
+
+#endif

sw/uart.s

@@ -0,0 +1,29 @@
+.include "io.inc65"
+
+.importzp sp, sreg
+
+.export _uart_txb
+.export _uart_txb_block
+.export _uart_status
+
+.autoimport	on
+
+.code
+
+; @in A: byte to transmit
+; Transmits a byte over the UART
+_uart_txb:
+        sta UART_TXB        ; Just write value, don't wait
+        rts
+
+_uart_txb_block:
+        sta UART_TXB        ; Write value
+@1:     lda UART_STATUS     ; Wait for status[0] to be 0
+        bit #$01
+        bne @1
+        rts
+
+_uart_status:
+        lda UART_STATUS
+        ldx #$00
+        rts