Add uart_rxb

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

sw/irq.c

@@ -2,12 +2,14 @@
 #include <stdint.h>
 
 #include "interrupt.h"
+#include "uart.h"
 
 // This is defined in main.c
 void puts(const char* s);
 
 void handle_irq() {
     uint8_t status;
+    char c;
 
     puts("Interrupt Detected!\n");
 
@@ -19,6 +21,10 @@ void handle_irq() {
     }
     if (status & UART) {
         puts("UART Interrupt!\n");
+        c = uart_rxb();
+        puts("Received: ");
+        puts(&c);
+        puts("\n");
         irq_set_status(status & ~UART);
     }
 }