3rdparty/Makefile.inc could only be used when included from the primary
makefile passed to make. It could not be used directly, or included from a
makefile that is itself included. This was due to counting from the left of
$(MAKEFILE_LIST) instead of using the last element.
Since each include directive appends to $(MAKEFILE_LIST), when using it to
determine $(THIRDPARTY_DIR), we need to use a simply-expanded variable.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
(Reapplying changes as one commit on top of development post codestyle change instead of rewriting old branch)
Signed-off-by: Aditya Deshpande <aditya.deshpande@arm.com>
