modules: add support for soft module dependencies

Additional and optional dependencies not found while building the kernel and
modules, can now be declared explicitly.

Signed-off-by: Andreas Robinson <andr345@gmail.com>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>