cmd: introduce 'write' command

It's almost no extra code to hook up a buddy to the 'read' command. In
fact, since the command is passed its own 'struct cmd_tbl', we can use
the exact same callback, and let it figure out for itself whether it
was invoked as "read" or "write".

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>

cmd/Makefile

@@ -140,6 +140,7 @@ obj-$(CONFIG_CMD_PXE) += pxe.o
 obj-$(CONFIG_CMD_WOL) += wol.o
 obj-$(CONFIG_CMD_QFW) += qfw.o
 obj-$(CONFIG_CMD_READ) += read.o
+obj-$(CONFIG_CMD_WRITE) += read.o
 obj-$(CONFIG_CMD_REGINFO) += reginfo.o
 obj-$(CONFIG_CMD_REISER) += reiser.o
 obj-$(CONFIG_CMD_REMOTEPROC) += remoteproc.o