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/README b/README
index 4f16240..22fd6b7 100644
--- a/README
+++ b/README
@@ -807,9 +807,11 @@
 		CONFIG_CMD_EEPROM	* EEPROM read/write support
 		CONFIG_CMD_ELF		* bootelf, bootvx
 		CONFIG_CMD_EXPORTENV	* export the environment
+		CONFIG_CMD_EXT2		* ext2 command support
+		CONFIG_CMD_EXT4		* ext4 command support
 		CONFIG_CMD_SAVEENV	  saveenv
 		CONFIG_CMD_FDC		* Floppy Disk Support
-		CONFIG_CMD_FAT		* FAT partition support
+		CONFIG_CMD_FAT		* FAT command support
 		CONFIG_CMD_FDOS		* Dos diskette Support
 		CONFIG_CMD_FLASH	  flinfo, erase, protect
 		CONFIG_CMD_FPGA		  FPGA device initialization support