Make the STM32 Makefile.includes usable from outside the examples tree

The Makefile.includes contain a hardcoded ../../../../../libopencm3
path for the TOOLCHAIN_DIR variable. They also contained another copy
of this hardcoded path, that is now generated from $TOOLCHAIN_DIR.

This allows to have a symbolic link to a Makefile.include in an
out-of-tree project and reuse the Makefile infrastructure.