build: support KNI cross-compilation

build: support KNI cross-compilation

The KNI linux module is using a custom target for building, which
doesn't take into account any cross compilation arguments. The arguments
in question are ARCH, CROSS_COMPILE (for gcc, clang) and CC, LD (for
clang). Get those from the cross file and pass them to the custom
target.

The user supplied path may not contain the 'build' directory, such as
when using cross-compiled headers, so only append that in the default
case (when no path is supplied in native builds) and use the unmodified
path from the user otherwise. Also modify the install path accordingly.

Signed-off-by: Juraj Linkeš <juraj.linkes@pantheon.tech>
Reviewed-by: Bruce Richardson <bruce.richardson@intel.com>