fs: separate CONFIG_FS_{FAT, EXT4} from CONFIG_CMD_{FAT, EXT*}

This makes the FAT and ext4 filesystem implementations build if
CONFIG_FS_{FAT,EXT4} are defined, rather than basing the build on
whether CONFIG_CMD_{FAT,EXT*} are defined. This will allow the
filesystems to be built separately from the filesystem-specific commands
that use them. This paves the way for the creation of filesystem-generic
commands that used the filesystems, without requiring the filesystem-
specific commands.

Minor documentation changes are made for this change.

The new config options are automatically selected by the old config
options to retain backwards-compatibility.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
diff --git a/include/ext4fs.h b/include/ext4fs.h
index 23298fc..3b59d15 100644
--- a/include/ext4fs.h
+++ b/include/ext4fs.h
@@ -116,7 +116,7 @@
 extern struct ext2_data *ext4fs_root;
 extern struct ext2fs_node *ext4fs_file;
 
-#if defined(CONFIG_CMD_EXT4_WRITE)
+#if defined(CONFIG_EXT4_WRITE)
 extern struct ext2_inode *g_parent_inode;
 extern int gd_index;
 extern int gindex;