Better support for external AVR_LIBC install.
In the Arduino IDE, all AVR tools are together in one bin directory.  In most external installs, the AVR-BINUTILS package provides most tools in one directory, and AVR-GCC provides the cc and cxx compilers in another.  AVRDUDE is installed in yet another directory. Arduino-mk now supports selectively providing some of these directories.  You can use your own binutils and gcc while still using the bundled avrdude.