arm: async echo app

arm/sleep.h

@@ -0,0 +1,10 @@
+#pragma once
+
+inline void sleep(int ms) {
+    for (int i = 0; i < ms; i++) {
+        // sleep for 1 ms
+        for (int j = 0; j < 22000; j++) {
+            asm("");
+        }
+    }
+}