libfdt: Tidy up pylibfdt build rule

At present the build rule for pylibfdt depends on _libfdt.so but modern
Python versions add a different suffix to the output file, resulting in
something like _libfdt.cpython-38-x86_64-linux-gnu.so

The result is that pylibfdt is rebuilt every time.

Rename the file the standard name so that the rule works correctly. Also
add libfdt.py to the dependencies, so that file is always created if
missing.
Signed-off-by: NSimon Glass <sjg@chromium.org>

scripts/dtc/pylibfdt/Makefile

@@ -23,12 +23,18 @@ quiet_cmd_pymod = PYMOD   $@
 		SWIG_OPTS="-I$(LIBFDT_srcdir) -I$(LIBFDT_srcdir)/.." \
 		$(PYTHON3) $< --quiet build_ext --inplace
 
-$(obj)/_libfdt.so: $(src)/setup.py $(PYLIBFDT_srcs) FORCE
+rebuild: $(src)/setup.py $(PYLIBFDT_srcs)
 	@# Remove the library since otherwise Python doesn't seem to regenerate
 	@# the libfdt.py file if it is missing.
-	rm -f $(obj)/_libfdt*.so
+	@rm -f $(obj)/_libfdt*.so
 	$(call if_changed,pymod)
+	@# Rename the file to _libfdt.so so this Makefile doesn't run every time
+	@if [ ! -e $(obj)/_libfdt.so ]; then \
+		mv $(obj)/_libfdt*.so $(obj)/_libfdt.so; \
+	fi
 
-always += _libfdt.so
+$(obj)/_libfdt.so $(obj)/libfdt.py &: rebuild
+
+always += _libfdt.so libfdt.py
 
 clean-files += libfdt.i _libfdt.so libfdt.py libfdt_wrap.c