kconfig: announce removal of oldnoconfig if used

The 'oldnoconfig' is really confusing due to its counter-intuitive name.
It was renamed by commit fb16d891 ("kconfig: replace 'oldnoconfig'
with 'olddefconfig', and keep the old name as an alias").

The 'oldnoconfig' has been kept as an alias for enough period of time,
and finally I am planning to remove it.  I will give people a little
more time for migration.  Meanwhile, the following message will be
displayed if oldnoconfig is used.

    WARNING: "oldnoconfig" target will be removed after Linux 4.19
              Please use "olddefconfig" instead, which is an alias.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Ulf Magnusson <ulfalizer@gmail.com>

scripts/kconfig/Makefile

@@ -92,6 +92,8 @@ PHONY += oldnoconfig savedefconfig defconfig
 # on its behavior (sets new symbols to their default value but not 'n') with the
 # counter-intuitive name.
 oldnoconfig: olddefconfig
+	@echo "  WARNING: \"oldnoconfig\" target will be removed after Linux 4.19"
+	@echo "            Please use \"olddefconfig\" instead, which is an alias."
 
 savedefconfig: $(obj)/conf
 	$< $(silent) --$@=defconfig $(Kconfig)