uart: add ledctrl app

uart/ledctrl.c

@@ -0,0 +1,24 @@
+#include "led.h"
+#include "uart.h"
+
+int main() {
+  int c;
+  int led = 0;
+
+  while (1) {
+    led_write(1 << led);
+    c = uart_read();
+
+    if (c == 'a') {
+      led++;
+    } else if (c == 'd') {
+      led--;
+    }
+
+    if (led == 8) {
+      led = 0;
+    } else if (led < 0) {
+      led = 7;
+    }
+  }
+}