kernel.bbclass: enable a link for external module building

kernel.bbclass: enable a link for external module building

Even though the kernel-dev package provides the required support for
building external kernel modules on the target, some commonly used scripts
and utilities fail as they are not finding the kernel module build support
files at the desired location.

Create the /lib/modules/<kernel-version>/build link on target pointing to
the sources provided by the kernel-dev package, to fix the issue.

Fixes Bug:
[YOCTO #2968]

(From OE-Core rev: aafa4bc896eb944aa4fc406807dd7e02f4b9b7ba)

Signed-off-by: Nitin A Kamble <nitin.a.kamble@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>