Added ZFS and SPL build hooks for building kernel module package

Please notice that this only builds and creates the kernel modules
package. The userland binaries are partly independent of the kernel
modules so they should be installed as well. Until then, the modules
can already be tested.

To compile the ZFS and SPL kernel modules, the follow packages are
required:

build-essential libtool automake autogen libz-dev uuid-dev rpm alien

The /usr/local/bin/rpm busybox symlink should also be removed or
else the wrong rpm binary will be used.

An additional post-build.d hook will be committed in the xbian
repository. The ZFS and SPL kernel modules can only be compiled
once the kernel itself is done building.