[modules] use VPATH to make it easier to load external modules

With the use of VPATH, external modules can more easily define the list of files to compile using the xml node rather than raw makefile section with global paths.
Also add some options to add custom compile/link flags.
This should allow to improve the use of vision modules for ardrone2 in particular.

Compilation is tested on bare metal, chibios and ardrone autopilots