pylibfdt: Use common PREFIX variable

For no particularly good reason, the install target for the Python library
uses a different PREFIX variable to give the installation destination
to the rest of dtc & libfdt.  Make it use the same one.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

pylibfdt/Makefile.pylibfdt

@@ -21,7 +21,7 @@ $(PYMODULE): $(PYLIBFDT_srcs) $(LIBFDT_archive) $(SETUP) $(VERSION_FILE)
 
 install_pylibfdt: $(PYMODULE)
 	@$(VECHO) INSTALL-PYLIB
-	$(PYTHON) $(SETUP) $(SETUPFLAGS) install $(if $(SETUP_PREFIX),--prefix=$(SETUP_PREFIX))
+	$(PYTHON) $(SETUP) $(SETUPFLAGS) install --prefix=$(PREFIX)
 
 pylibfdt_clean:
 	@$(VECHO) CLEAN "(pylibfdt)"