[ELinOS] Best effort to load system libraries in case of incomplete env

[ELinOS] Best effort to load system libraries in case of incomplete env

So far elinos.py was assuming that the whole ELinOS environment was
around to find the system libraries; if some environment variables
were missing, the script would just abort.

This was a bit extreme. It is possible to do better than that: to get
the core system libraries, one doesn't need to have a full environment
but just the path to the CDK. The path to kernel project is only
needed for the optional Xenomai libs.

gdb/ChangeLog:

	* system-gdbinit/elinos.py (get_elinos_environment): Return an
	incomplete dictionnary instead of None in case of missing
	environment variables.
	(elinos_init): in case of an incomplete environment, best
	effort to load system libraries instead of abort.