The file(s) being removed may not exist, but make shouldn't fail because
of it:
$ make clean
rm ./firmware/*.uf2
rm: cannot remove './firmware/*.uf2': No such file or directory
make: *** [Makefile:10: clean] Error 1
$ echo $?
2
this also removes leading ./ from path in clean, and assumes that `make
clean` is being run from the top level repo dir
