added support for the LoFive R1 (#17)

* added support for the LoFive R1

* corrected src/clock.rs

* added CI rule for `board-lofive-r1` feature

* added linkerscript for the LoFive R1

Note that this linkerscript assumes the bootloader is installed.
Instructions on how to do that can be found here:

https://github.com/mwelling/lofive/wiki#programming-to-lofive-r1-using-the-new-sdk

It'll "work" without the bootloader, but user code won't start after a
reset.