pylibfdt: Use Python 2 in Makefile

pylibfdt needs Python 2 to build.
Replace $(PYTHON) with $(PYTHON2) in pylibfdt Makefile
to ensure Python 2 is used to build it.

This fixes build on systems where Python 3 is the default version
of the "python" interpreter.
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/scripts/dtc/pylibfdt/Makefile b/scripts/dtc/pylibfdt/Makefile
index c769d7d..15e66ad 100644
--- a/scripts/dtc/pylibfdt/Makefile
+++ b/scripts/dtc/pylibfdt/Makefile
@@ -21,7 +21,7 @@
 		CPPFLAGS="$(HOSTCFLAGS) -I$(LIBFDT_srcdir)" OBJDIR=$(obj) \
 		SOURCES="$(PYLIBFDT_srcs)" \
 		SWIG_OPTS="-I$(LIBFDT_srcdir) -I$(LIBFDT_srcdir)/.." \
-		$(PYTHON) $< --quiet build_ext --inplace
+		$(PYTHON2) $< --quiet build_ext --inplace
 
 $(obj)/_libfdt.so: $(src)/setup.py $(PYLIBFDT_srcs) FORCE
 	$(call if_changed,pymod)