build system: do not use GNU-isms in find

Signed-off-by: Chris Rees <utisoft@gmail.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

Makefile

@@ -1008,8 +1008,8 @@ $(mrproper-dirs):
 mrproper: clean archmrproper $(mrproper-dirs)
 	$(call cmd,rmdirs)
 	$(call cmd,rmfiles)
-	@find -name Config.src | sed 's/.src$$/.in/' | xargs -r rm -f
-	@find -name Kbuild.src | sed 's/.src$$//' | xargs -r rm -f
+	@find . -name Config.src | sed 's/.src$$/.in/' | xargs -r rm -f
+	@find . -name Kbuild.src | sed 's/.src$$//' | xargs -r rm -f
 
 # distclean
 #