Makefile: have "make distclean" remove CHANGELOG

Ensure that "make distclean" deletes an existing CHANGELOG file.
Signed-off-by: NRobert P. J. Day <rpjday@crashcourse.ca>

Makefile

@@ -1795,7 +1795,7 @@ distclean: mrproper
 		-o -name '.*.rej' -o -name '*%' -o -name 'core' \
 		-o -name '*.pyc' \) \
 		-type f -print | xargs rm -f
-	@rm -f boards.cfg
+	@rm -f boards.cfg CHANGELOG
 
 backup:
 	F=`basename $(srctree)` ; cd .. ; \