Check for Pro Micro serial port automatically

[nicinabox]

A small improvement to #1036 that checks for the new serial port automatically instead of having the user manually continue.

[jackhumbert]

Nice - thanks!

[NoahAndrews]

Nice! I have a Pro Micro now. I should look into getting similar functionality on Windows, which we can also use in the flasher.

[nicinabox]

@NoahAndrews That would be awesome. I see a lot of folks struggling on Windows. Seems like with the new addition of bash on Windows that could be more unix-like?

[NoahAndrews]

I really love the new BASH on Windows, but it doesn't support serial ports yet, so using that environment to flash the Pro Micro is out. :/

[nicinabox]

Ahhh good to know. That's a bummer.

[NoahAndrews]

There's a pretty good chance I won't bother implementing this in the Makefile. Right now, Windows doesn't have a good native build environment. We're mostly pushing people to build the firmware in WSL (aka BASH on Ubuntu on Windows) or in some sort of virtual environment (Virtualbox, Docker, Vagrant). For Virtualbox (and maybe Vagrant or Docker), it's possible to just connect the keyboard to the VM and flash it from there, but for the others it's best to use the QMK Firmware Flasher. It's not worth bothering with adding Windows support for this to the Makefile if there isn't even a good, recommended build environment that could make use of it.

Adding support to the QMK Firmware Flasher is definitely higher priority.