Added check to see if TOSDIR is provided
Switched the search order: targets are now first

This helps with out-of-tree tinyos builds if there is a separate tinyos
tree. I've ran into a case where a .extra target in tinyos-main was
overriding my .target file and the wrong files were being compiled.
Removed $(MAKERULES)

All makefiles now use Makefile.include with a relative path and not the
environment variable $(MAKERULES).