Add license.txt

[lesto]

What is the license used in this code? is quite hard to find out, looking at the firmware header seems there is no limitation (BSD-like?), but is that true for all the code base?

[bkueng]

I think it's BSD. @LorenzMeier can you confirm?

[LorenzMeier]

We can add a license file and it will reflect BSD. However, there are infrastructure libraries and derived file in use here that have their own license.