User callbacks template updated

Andrea Zoppi
Documentation updated

Andrea Zoppi
Makefiles reworked

Makefile scripts for uROSnode were moved to the "mk/" folder. The main incldue
file is "uros.mk" (instead of "common.mk"), and needs to be incldued before
those for low-level drivers. The latter were split into atomic makefiles
("chibios+lwip.mk" are now "chibios.mk" and "lwip.mk").

The demo Makefiles can now optionally generate static analysis for C files.
Inside the "build/stan/" folder, GNU cflow will generate *.cflow reports of
direct calls for C files, while GCC will create *.su files for static stack
usage. These files may become handy to approximate the worst-case stack usage of
the threads.

Andrea Zoppi