cmd: add clone command

This patch adds a feature for block device cloning similar to dd
command, this should be useful for boot-strapping a device where
usb gadget or networking is not available. For instance one can
clone a factory image into a blank emmc from an external sd card.

Signed-off-by: John Chau <john@harmon.hk>
diff --git a/cmd/Kconfig b/cmd/Kconfig
index d7136b0..e111764 100644
--- a/cmd/Kconfig
+++ b/cmd/Kconfig
@@ -1148,6 +1148,14 @@
 
 endif
 
+config CMD_CLONE
+	bool "clone"
+	depends on BLK
+	help
+	  Enable storage cloning over block devices, useful for
+	  initial flashing by external block device without network
+	  or usb support.
+
 config CMD_MTD
 	bool "mtd"
 	depends on MTD