This is a wrapper for apps/Make.defs or nuttx/Make.defs
Since apps/Make.defs include nuttx/Make.defs, we check if it the
apps dir is available and include it, otherwise we just include
nuttx/Make.defs and keep the old behavior.
This allows EXTRA_LIBS and some other system-wide tweaks be
applied in any apps and make it work with the nuttx Makefiles.
Marco Casaroli