kconfig: use $(PERL) in Makefile

The top Makefile defines and exports the variable 'PERL'. Use it in
case somebody wants to specify a particular version of perl from the
command line.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

scripts/kconfig/Makefile

@@ -47,7 +47,7 @@ build_gconfig: $(obj)/gconf
 build_xconfig: $(obj)/qconf
 
 localyesconfig localmodconfig: $(obj)/conf
-	$(Q)perl $(srctree)/$(src)/streamline_config.pl --$@ $(srctree) $(Kconfig) > .tmp.config
+	$(Q)$(PERL) $(srctree)/$(src)/streamline_config.pl --$@ $(srctree) $(Kconfig) > .tmp.config
 	$(Q)if [ -f .config ]; then 				\
 		cmp -s .tmp.config .config ||			\
 		(mv -f .config .config.old.1;			\