linux-fusion: add dependency on !BR2_KERNEL_none

linux-fusion is composed of a kernel module, and linux-fusion.mk uses
several variable definitions that only exist when Buildroot compiles a
kernel.

We also add the same dependency to BR2_PACKAGE_DIRECTFB_MULTI, which
relies on linux-fusion.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>