libiio: 0.20 -> 0.21

* Format with nixpkgs-format.
* Reorder the attribute to be more in line with convention.
* Use simpler git ref.
* Hardcode paths within bindings using patch (the replaced value actually changed in this version and without this clean-up, it would have gone unnoticed.)
* Let the build script install the python bindings instead of doing it manually (required the same hardcoding as above in the setup.py.)
* Use cmakeFlags attribute with placeholder instead of doing that in preConfigure.
* Move udev rules to lib since etc is meant for admins.
* License is actually lgpl2.1+.