proper use of connection exec during update

lib/tessel/update.js

@@ -38,36 +38,34 @@ Tessel.prototype.update = function(newImage) {
 Tessel.prototype.updateOpenWRT = function(image) {
   logs.info('Updating OpenWRT (1/2)');
 
-  // Write the new image to a file in the /tmp dir
-  return this.connection.exec(commands.openStdinToFile(updatePath))
-    .then((remoteProc) => {
-      return new Promise(function(resolve) {
-        logs.info('Transferring image of size', (image.length / 1e6).toFixed(2), 'MB');
-        // When we finish writing the image
-        remoteProc.once('close', resolve);
-        // Write the image
-        remoteProc.stdin.end(image);
-      });
-    })
-    .then(() => {
+  return new Promise((resolve, reject) => {
+    // Write the new image to a file in the /tmp dir
+    this.connection.exec(commands.openStdinToFile(updatePath), (err, remoteProc) => {
+      if (err) {
+        return reject(err);
+      }
+
+      logs.info('Transferring image of size', (image.length / 1e6).toFixed(2), 'MB');
+      // When we finish writing the image
+      remoteProc.once('close', resolve);
+      // Write the image
+      remoteProc.stdin.end(image);
+    });
+  })
+  .then(() => {
+    return new Promise((resolve) => {
       // Begin the sysupgrade
       logs.info('Starting OpenWRT update. Please do not remove power from Tessel.');
       // The USBDaemon will cut out or the SSH command will close
-      return this.connection.exec(commands.sysupgrade(updatePath));
-    })
-    .then((remoteProc) => {
-      // This is the best way I've figured out to tell
-      // when the update is complete. I realize that tying completion
-      // of the CLI command to random text output by OpenWRT might not
-      // be the best thing...
-      return new Promise(function(resolve) {
+      this.connection.exec(commands.sysupgrade(updatePath), (err, remoteProc) => {
         remoteProc.stdout.on('data', function(d) {
           if (d.toString().includes('Upgrade completed')) {
             resolve();
           }
         });
       });
     });
+  });
 };
 
 Tessel.prototype.updateFirmware = function(image) {