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>
diff --git a/cmd/Kconfig b/cmd/Kconfig
index a351283..ba5ec69 100644
--- a/cmd/Kconfig
+++ b/cmd/Kconfig
@@ -1562,6 +1562,11 @@
 	help
 	  This provides commands to control the watchdog timer devices.
 
+config CMD_WRITE
+	bool "write - Write binary data to a partition"
+	help
+	  Provides low-level write access to a partition.
+
 config CMD_AXI
 	bool "axi"
 	depends on AXI