kbuild: remove auto.conf from prerequisite of phony targets

The top-level Makefile adds include/config/auto.conf as
prerequisites of 'scripts', 'prepare1', etc.

They were needed to terminate the build when include/config/auto.conf
is missing.

Now that the inclusion of include/config/auto.conf is mandatory
in the top-level Makefile if dot-config is 1 (Note 'include' directive
is used instead of '-include').

Make terminates the build by itself if it fails to create or update
include/config/auto.conf so we are sure that include/config/auto.conf
exists in the very first stage of make.

I am still keeping include/config/auto.conf as the prerequisite of
%/modules.builtin because modules.builtin is a real file.  According
to commit a6c36632 ("kbuild: Do not unnecessarily regenerate
modules.builtin"), it is intentional to compare time-stamps between
%/modules.builtin and include/config/auto.conf .  I moved tristate.conf
here because it is only included from scripts/Makefile.modbuiltin.

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