mk: fix custom kernel directory name

mk: fix custom kernel directory name

When building dpdk with different kernel headers by specifying
RTE_KERNELDIR igb_uio is compiled to directory with a name of the
version of kernel thats running on the system instead of the one that
dpdk is actually compiled against. Fixed by replacing hardcoded value
with value from RTE_KERNELDIR.

Fixes: 3967af352aeb ("mk: install kernel modules")
Cc: stable@dpdk.org
Signed-off-by: Herakliusz Lipiec <herakliusz.lipiec@intel.com>
Acked-by: Thomas Monjalon <thomas@monjalon.net>