Add CONFIG_OVERLAYFS_FS to check-config.sh

Also, added some slight adjustment to the AUFS_FS output/note to make it more clear what it applies to.

Example output:
```console
$ ./contrib/check-config.sh
info: reading kernel config from /proc/config.gz ...

Generally Necessary:
- cgroup hierarchy: properly mounted [/sys/fs/cgroup]
- CONFIG_NAMESPACES: enabled
- CONFIG_NET_NS: enabled
- CONFIG_PID_NS: enabled
- CONFIG_IPC_NS: enabled
- CONFIG_UTS_NS: enabled
- CONFIG_DEVPTS_MULTIPLE_INSTANCES: enabled
- CONFIG_CGROUPS: enabled
- CONFIG_CGROUP_CPUACCT: enabled
- CONFIG_CGROUP_DEVICE: enabled
- CONFIG_CGROUP_FREEZER: enabled
- CONFIG_CGROUP_SCHED: enabled
- CONFIG_MACVLAN: enabled
- CONFIG_VETH: enabled
- CONFIG_BRIDGE: enabled
- CONFIG_NF_NAT_IPV4: enabled
- CONFIG_IP_NF_FILTER: enabled
- CONFIG_IP_NF_TARGET_MASQUERADE: enabled
- CONFIG_NETFILTER_XT_MATCH_ADDRTYPE: enabled
- CONFIG_NETFILTER_XT_MATCH_CONNTRACK: enabled
- CONFIG_NF_NAT: enabled
- CONFIG_NF_NAT_NEEDED: enabled

Optional Features:
- CONFIG_MEMCG_SWAP: enabled
- CONFIG_RESOURCE_COUNTERS: enabled
- CONFIG_CGROUP_PERF: missing
- Storage Drivers:
  - "aufs":
    - CONFIG_AUFS_FS: missing
      (note that some kernels include AUFS patches but not the AUFS_FS flag)
    - CONFIG_EXT4_FS_POSIX_ACL: enabled
    - CONFIG_EXT4_FS_SECURITY: enabled
  - "btrfs":
    - CONFIG_BTRFS_FS: enabled
  - "devicemapper":
    - CONFIG_BLK_DEV_DM: enabled
    - CONFIG_DM_THIN_PROVISIONING: enabled
    - CONFIG_EXT4_FS: enabled
    - CONFIG_EXT4_FS_POSIX_ACL: enabled
    - CONFIG_EXT4_FS_SECURITY: enabled
  - "overlayfs":
    - CONFIG_OVERLAYFS_FS: missing
```

Signed-off-by: Andrew Page <admwiggin@gmail.com>

contrib/check-config.sh

@@ -153,16 +153,20 @@ check_flags "${flags[@]}"
 echo '- Storage Drivers:'
 {
 	echo '- "'$(wrap_color 'aufs' blue)'":'
-	check_flags AUFS_FS EXT4_FS_POSIX_ACL EXT4_FS_SECURITY | sed 's/^/  /'
+	check_flags AUFS_FS | sed 's/^/  /'
 	if ! is_set AUFS_FS && grep -q aufs /proc/filesystems; then
 		echo "    $(wrap_color '(note that some kernels include AUFS patches but not the AUFS_FS flag)' bold black)"
 	fi
+	check_flags EXT4_FS_POSIX_ACL EXT4_FS_SECURITY | sed 's/^/  /'
 
 	echo '- "'$(wrap_color 'btrfs' blue)'":'
 	check_flags BTRFS_FS | sed 's/^/  /'
 
 	echo '- "'$(wrap_color 'devicemapper' blue)'":'
 	check_flags BLK_DEV_DM DM_THIN_PROVISIONING EXT4_FS EXT4_FS_POSIX_ACL EXT4_FS_SECURITY | sed 's/^/  /'
+
+	echo '- "'$(wrap_color 'overlayfs' blue)'":'
+	check_flags OVERLAYFS_FS | sed 's/^/  /'
 } | sed 's/^/  /'
 echo