Expose usb.sendUsbPacketsByCallback() and use it to vastly simplify blink-test-led.js and led-pwm.js

usb/blink-test-led.js

@@ -1,25 +1,15 @@
 #!/usr/bin/env node
-'use strict';
-
 let uhk = require('./uhk');
-let [endpointIn, endpointOut] = uhk.getUsbEndpoints();
 
-var state = 1;
+let delayCycle = true;
+let areLedsEnabled = true;
 
-setInterval(function() {
-    state = state ? 0 : 1
-    console.log('Sending ', state);
-    endpointOut.transfer(new Buffer([uhk.usbCommands.setTestLed, state]), function(err) {
-        if (err) {
-            console.error("USB error: %s", err);
-            process.exit(1);
-        }
-        endpointIn.transfer(64, function(err2, receivedBuffer) {
-            if (err2) {
-                console.error("USB error: %s", err2);
-                process.exit(2);
-            }
-            console.log('Received', uhk.bufferToString(receivedBuffer));
-        })
-    });
-}, 500)
+uhk.sendUsbPacketsByCallback(() => {
+    delayCycle = !delayCycle;
+    if (delayCycle) {
+        return new uhk.DelayMs(500);
+    } else {
+        areLedsEnabled = !areLedsEnabled;
+        return new Buffer([uhk.usbCommands.setTestLed, areLedsEnabled ? 1 : 0]);
+    }
+});