kconfig: invoke silentoldconfig if spl, tpl/.config is updated

When spl/.config is updated by "make spl/menuconfig" or friends,
spl/include/config/auto.conf, spl/include/generated/autoconf.h
and some other files must be updated by "make silentoldconfig".

There is no hook for SPL in the top Makefile, so this commit
touches .config when spl/.config is updated to invoke silentoldconfig.
Likewise for TPL.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>