Fix Kbuild invocation

The make command invoking the Linux build system was relying on the src
variable, to be able to put build artifacts into a dedicated directory.
However, this use case is not officially supported by the kernel build
system and stopped working for recent kernel versions. Let's move to the
offical documented solution instead.

Signed-off-by: Balint Dobszay <balint.dobszay@arm.com>
Change-Id: Ie8d0078a2324410bb848622ff30575af502220b9