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/fs/fat/Makefile b/fs/fat/Makefile
index 02e6881..969715b 100644
--- a/fs/fat/Makefile
+++ b/fs/fat/Makefile
@@ -24,11 +24,11 @@
 LIB	= $(obj)libfat.o
 
 AOBJS	=
-COBJS-$(CONFIG_CMD_FAT)	:= fat.o
+COBJS-$(CONFIG_FS_FAT)	:= fat.o
 COBJS-$(CONFIG_FAT_WRITE):= fat_write.o
 
 ifndef CONFIG_SPL_BUILD
-COBJS-$(CONFIG_CMD_FAT)	+= file.o
+COBJS-$(CONFIG_FS_FAT)	+= file.o
 endif
 
 SRCS	:= $(AOBJS:.o=.S) $(COBJS-y:.o=.c)