build: Fix KDIR again

build: Fix KDIR again

KDIR is not related to the what we configure in kmod's build. It's only
used in kmod to locate where the distro's kernel source/headers is,
which may be different from what we are configuring the (under
development) kmod with.

Remove the setting from meson/autotools and figure it out inside the
module-playground Makefile what should be used. For advanced use cases,
KDIR= can be passed to override the location.

For our own tests, which includes testing with a different module_directory,
scripts/setup-rootfs.sh will copy the module to the desired location
according to the map defined in the script.

Fixes: 27ff72732666 ("testsuite: correct the default KDIR")
Signed-off-by: Lucas De Marchi <lucas.de.marchi@gmail.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Link: https://github.com/kmod-project/kmod/pull/125