cmd: add sdp command

Add a new command to start USB Serial Download Protocol (SDP)
state machine.

Signed-off-by: Stefan Agner <stefan.agner@toradex.com>
Reviewed-by: Ɓukasz Majewski <lukma@denx.de>
Reviewed-by: Stefano Babic <sbabic@denx.de>
diff --git a/cmd/Kconfig b/cmd/Kconfig
index 42d955c..d6d130e 100644
--- a/cmd/Kconfig
+++ b/cmd/Kconfig
@@ -895,6 +895,13 @@
 	help
 	  USB support.
 
+config CMD_USB_SDP
+	bool "sdp"
+	select USB_FUNCTION_SDP
+	help
+	  Enables the command "sdp" which is used to have U-Boot emulating the
+	  Serial Download Protocol (SDP) via USB.
+
 config CMD_USB_MASS_STORAGE
 	bool "UMS usb mass storage"
 	help