package/pkg-kernel-module.mk: module build fails

Observed the following when building cryptodev-linux:
>>> cryptodev-linux 1.7 Building kernel module(s)
grep: /.config: No such file or directory
ERROR: Kernel does not support loadable modules

Fix LINUX_DIR reference when determining if kernel configuration
supports loadable modules.

Signed-off-by: Doug Kehn <rdkehn@yahoo.com>
Reviewed-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>