busybox: disable support for mounting NFS file systems on Linux < 2.6.23

busybox: disable support for mounting NFS file systems on Linux < 2.6.23

The busybox CONFIG_FEATURE_MOUNT_NFS config option is described as:

  Enable mounting of NFS file systems on Linux kernels prior
  to version 2.6.23. Note that in this case mounting of NFS
  over IPv6 will not be possible.

Since OE-core sets OLDEST_KERNEL = "2.6.32", CONFIG_FEATURE_MOUNT_NFS
is not required in the default busybox defconfig.

Signed-off-by: Andre McCurdy <armccurdy@gmail.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>