kernel: fix cross-build of Linux modules with meson

kernel: fix cross-build of Linux modules with meson

When cross-compiling, if no kernel_dir was specified, then the kernel
modules were still being compiled for the build machine. Fix this by
only building modules on cross-compile when we have a kernel_dir value
set. Print out a message indicating why we are skipping kernel
compilation, and in case that the headers for kernel compile are not
found, print a warning instead of erroring out.

Fixes: a52f4574f798 ("igb_uio: build with meson")

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Luca Boccassi <bluca@debian.org>