Allow to build just the binaries or just the libraries from the top level Makefile.
Removed reference to CC65_HOME.

Now that we're pretty much independent from CC65_HOME
it doesn't make sense anymore to look for it in the Makefiles.

So rather dynamically check if there are "local" binaries in the
../bin directory. If there are "local" binaries use them - and
otherwise rely on "installed" binaries found in the path.