kbuild: slim down package for building external modules

Exclude directories and files unnecessary for building external modules:

 - include/config/  (except include/config/{auto.conf,kernel.release})
 - scripts/atomic/
 - scripts/dtc/
 - scripts/kconfig/
 - scripts/mod/mk_elfconfig
 - scripts/package/
 - scripts/unifdef
 - .config
 - *.o
 - .*.cmd

Avoid copying files twice for the following directories:

 - include/generated/
 - arch/*/include/generated/

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>