meta-phosphor: add nfs distro feature

Add include files for NFS distro features, which enables NFS kernel
configuration and utility to mount NFS from BMC.

Tested:
1. Check if CONFIG_NFS_* are enabled
$ zcat /proc/config.gz | grep NFS
2. Mount remote NFS folder
$ mount -t nfs 10.10.130.2:/tftpboot/ISO_file /tmp/mount_nfs

Change-Id: I015f4f6ed9c5b60573340233aee1f781aa0c573a
Signed-off-by: Thang Q. Nguyen <thang@os.amperecomputing.com>
diff --git a/meta-phosphor/conf/distro/include/nfs.inc b/meta-phosphor/conf/distro/include/nfs.inc
new file mode 100644
index 0000000..de28475
--- /dev/null
+++ b/meta-phosphor/conf/distro/include/nfs.inc
@@ -0,0 +1,2 @@
+DISTRO_FEATURES += "nfs"
+DISTROOVERRIDES .= ":df-nfs"