iso: enable Network Block Device support

Having NBD module support would enable some of the CSI projects to utilize
the NBD based persistent volume requests validation in their automated
testing suites.

Example, we need it at the ceph-csi project.

Signed-off-by: Prasanna Kumar Kalever <prasanna.kalever@redhat.com>

deploy/iso/minikube-iso/board/coreos/minikube/linux_defconfig

@@ -312,6 +312,7 @@ CONFIG_DEVTMPFS_MOUNT=y
 CONFIG_DEBUG_DEVRES=y
 CONFIG_CONNECTOR=y
 CONFIG_BLK_DEV_LOOP=y
+CONFIG_BLK_DEV_NBD=m
 CONFIG_VIRTIO_BLK=y
 CONFIG_BLK_DEV_RBD=m
 CONFIG_VMWARE_BALLOON=m