kbuild: set KBUILD_MODULES=1 all the time for single target %/

It is fine to set KBUILD_MODULES=1 when CONFIG_MODULES is disabled.
It is actually how "make allnoconfig all" works.

On the other hand, KBUILD_MODULES=1 is unneeded for the %.ko pattern.
It is just a matter of whether modules.order is generated or not.

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