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>