The toplevel directory is actually just ../..: the makefile commands
are executed in the subdirectory. $(PWD) earlier was wrong because it
comes from the shell, not from make. Looking up $(MAKEFILE_LIST) is
wrong because it indicates where the makefile is (make -f), not which
directory to work in (make -C).
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>