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>

fs/ext4/ext4fs.c

@@ -196,7 +196,7 @@ int ext4fs_read(char *buf, unsigned len)
 	return ext4fs_read_file(ext4fs_file, 0, len, buf);
 }
 
-#if defined(CONFIG_CMD_EXT4_WRITE)
+#if defined(CONFIG_EXT4_WRITE)
 static void ext4fs_update(void)
 {
 	short i;