SCons: Allow top level directory of EXTRAS able to contain SConscripts.

SCons: Allow top level directory of EXTRAS able to contain SConscripts.
The current EXTRAS will fail if the top level directory pointed to by EXTRAS
has a SConscript file in it.  We allow this by including the directory name
of the EXTRA in the build directory which prevents a clash between
src/SConscript and extra/SConscript. Maintain compatibility with older uses
of EXTRAS by adding a -I for each top level extra directory.