cmd_fat: add FAT write command

Once CONFIG_FAT_WRITE is defined, users can invoke 'fatwrite' command
that saves data in RAM as a FAT file.

This patch was originally part of

http://article.gmane.org/gmane.comp.boot-loaders.u-boot/121847

Signed-off-by: Donggeun Kim <dg77.kim@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Maximilian Schwerin <mvs@tigris.de>
Signed-off-by: Anatolij Gustschin <agust@denx.de>
diff --git a/README b/README
index b69a3b6..110eac6 100644
--- a/README
+++ b/README
@@ -1233,8 +1233,12 @@
 
 - FAT(File Allocation Table) filesystem write function support:
 		CONFIG_FAT_WRITE
-		Support for saving memory data as a file
-		in FAT formatted partition
+
+		Define this to enable support for saving memory data as a
+		file in FAT formatted partition.
+
+		This will also enable the command "fatwrite" enabling the
+		user to write files to FAT.
 
 - Keyboard Support:
 		CONFIG_ISA_KEYBOARD