kconfig: fix MODULES-related bug in case of no .config

There seems to be a kconfig bug due to MODULES not always being
evaluated if no .config is found. Take the following Kconfig as an
example:

config MODULES
	def_bool y

config FOO
	def_tristate m

With no .config, the following configuration is generated:

CONFIG_MODULES=y
CONFIG_FOO=y

With an empty .config, the following:

CONFIG_MODULES=y
CONFIG_FOO=m

Tristate choice statements can also exhibit the problem, due to having an
implicit rev_dep (select) containing "m".

The problem is that MODULES is never evaluted in conf_read_simple() unless
there's a .config. The following patch fixes this.

Signed-off-by: Ulf Magnusson <ulfalizer.lkml@gmail.com>
Reviewed-by: WANG Cong <xiyou.wangcong@gmail.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>