sdk: predefine kernel version magic

Kernel modules which are built using the SDK do not have access
to the .vermagic file in the Linux build dir, therefore the
generated .ipk metadata depends on kernel versions like
'3.10.49-1-unknown' which are not satisfiable by the kernel
meta package in the image.

Fix this problem by substituting 'unknown' with the current
version magic in include/kernel.mk when packing the SDK.

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>

SVN-Revision: 42590