Servo set angle doesn't wait for servo to power on

[ssande-fwd]

Describe the bug
Your program uses a servo and sets angle on button press. After you first download the program to the microbit, the first button press enables the servo but does not set it. The second button press sets it.

To Reproduce
Steps to reproduce the behavior:

1. Have your program set the angle to 90° on a button press.
2. Download the program to the microbit.
3. Observe the servo state using the jacdac web tools. Note that the servo is disabled.
4. Press the button. Observe that the servo is enabled but not set to 90°.
5. Press the button again. Observe that the servo is set to 90°.

Expected behavior
The servo is set to 90° on the first button press.

Additional context
Observed with Forward Education breakout board and Kittenbot Jacdaptor.

[ssande-fwd]

The issue is that internalSetAngle() calls this.setEnabled(true) but doesn't wait for the servo to become responsive before setting the value. If you add a 1 second timeout after this.setEnabled(true) then the set angle goes through. However, this is not an elegant solution. The servo will swing to whatever it's initialized to first and then swing again to the set value.

I wasn't able to find a way to poll for servo responsiveness rather than using a timeout.

Another thing that works is simply enabling the servo in on start as a MakeCode user. That's not really intuitive though.

https://github.com/microsoft/pxt-jacdac/blob/master/servo/client.ts#L105