linux: add blind kconfig option to require kernel modules

Currently, packages that need the kernel to have support for laodable
modules have two ways to require it:

  - either the use the kernel-module infra, which does it automatically,

  - or they do not use it, and they need to require it manually by
    setting the corresponding Makefile variable; however, they must only
    set it when they are actually enabled, which makes for a slightly
    cumbersome and ugly code, like:

        ifeq ($(BR2_PACKAGE_FOO),y)
        LINUX_NEEDS_MODULES = y
        endif

Introduce a new blind Kconfig option that packages can select to signify
they need kernel modules. That Kconfig option is then used to set the
Makefile variable.

It makes it cleaner:

  - code is simpler (one Kconfig line instead of a Makefile if-block,

  - this is handled at the Kconfig level, which is where we usually
    handle such dependencies.

Packages will be updated in follow-up commits.

Reported-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>