kbuild: pkg: grep include/config/auto.conf instead of $KCONFIG_CONFIG

This will be a little more efficient since unset CONFIG options are
stripped away from auto.conf, and we can hard-code the path to auto.conf
since it is never overridden.

include/config/kernel.release is generated before %pkg is run.
So, it is guaranteed auto.conf is up-to-date.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>