Make VERSION_BLINK_PIN optional

[cmaglie]

This may be useful for boards without a built-in led or if you simply want to disable this feature.

Also removed a redundant pinMode (the pin is already set as output inside strobeBlinkPin).

[soundanalogous]

I'd rather have blinkVersion() do nothing in this case to avoid the calls to delay. Add the compiler conditional inside blinkVersion() instead so it essentially does nothing if the LED pin is not specified. This should be safe since strobeBlinkPin() is private and is only called by blinkVersion().

[soundanalogous]

@cmaglie I've made an alternative version that avoids the delays in blinkVersion. However I'm not sure how the feature can be disabled as you requested, other than the user modifying Boards.h to undefine VERSION_BLINK_PIN for a particular board. Let me know if you have any ideas. I tried adding the following here

#if defined(VERSION_BLINK_PIN) && !defined(FIRMATA_SKIP_BLINK_VERSION)

and then defining FIRMATA_SKIP_BLINK_VERSION in a sketch before including Firmata.h but that doesn't work. I assume Firmata.h is compiled before any defines above it in a sketch are evaluated by the preprocessor. Perhaps there is something simple I am overlooking.