Support third party user installed boards

I added the [Sparkfun 32u4 boards](https://github.com/sparkfun/SF32u4_boards) to my arduino setup, but ino didn't find them. I don't really know python, but I gave it a stab in trying to get it to glob and find any board.txt files in subfolders of the hardware dir. Works on my machine. Any and all feedback welcome.

Probably could use some tests, but I wanted to open the PR up ASAP. Will investigate adding some :smile_cat: