kbuild: external modules shall not check config consistency

external modules needs include/linux/autoconf.h and include/config/auto.conf
but skip the integrity test of these. Even with a newer Kconfig file we
shall just proceed since external modules simply uses the kernel source and
shall not attempt to modify it.
Error out if a config fiel is missing since they are mandatory.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>