[BUILD] Add better autodetection of libopencm3 paths

In previous way was a bug, because in CLEAN library, there is no *.a files, so it cannot be detected that way.

With this commit, locm3 will be searched in:

./libopencm3 [the default if the user want use makefile.rules in his own project]
../../../../../libopencm3 [building in the examples]
$(TOOLCHAIN_DIR) [installed version in the toolchain path]

So it enables to the user that he may grab the Makefile.rules and copy it to his working directory and benefits same setting of compiler for library and his code. He may benefit from generic flashers and so on ...
[FIX] lm3s and lpc has another deep of makefiles