AVR recipe.ar.pattern backwards compatibility

Define archive_file_path in Arduino AVR Boards platform.txt to provide
backwards compatibility with IDE versions previous to 1.6.6. The
archive_file_path value set in platform.txt is overridden in IDE 1.6.6+.
This produces avr-ar commands identical to the previous behavior.

hardware/arduino/avr/platform.txt

@@ -62,6 +62,7 @@ recipe.cpp.o.pattern="{compiler.path}{compiler.cpp.cmd}" {compiler.cpp.flags} -m
 recipe.S.o.pattern="{compiler.path}{compiler.c.cmd}" {compiler.S.flags} -mmcu={build.mcu} -DF_CPU={build.f_cpu} -DARDUINO={runtime.ide.version} -DARDUINO_{build.board} -DARDUINO_ARCH_{build.arch} {compiler.S.extra_flags} {build.extra_flags} {includes} "{source_file}" -o "{object_file}"
 
 ## Create archives
+archive_file_path={build.path}/{archive_file}
 recipe.ar.pattern="{compiler.path}{compiler.ar.cmd}" {compiler.ar.flags} {compiler.ar.extra_flags} "{archive_file_path}" "{object_file}"
 
 ## Combine gc-sections, archives, and objects