linux-fusion: many fixes

linux-fusion: many fixes

This commit fixes several issues with linux-fusion so that it builds
properly with the current Buildroot:

 * Following the rework of the Linux kernel build process in
   Buildroot, a few adaptations were needed: depend on
   BR2_LINUX_KERNEL instead of !BR2_KERNEL_none, use
   $(LINUX26_VERSION_PROBED) to get the real kernel version, etc.

 * Add a LINUX_FUSION_BUILD_CMDS that actually does the build of
   linux-fusion.

 * Define SYSROOT for linux-fusion to TARGET_DIR so that at target
   installation, kernel modules are installed in $(TARGET_DIR)

 * Replace the staging installation by a simple one-liner that only
   installs the linux fusion header, needed for DirectFB to use the
   fusion kernel module

 * Fix the uninstallation macros. They have been misunderstood as
   hooks, while their are in fact normal macros.

 * Add a patch to fix a build failure in linux-fusion itself (missing
   <linux/sched.h> include in fusiondev.c)

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