cmd: Add dependency for ums command

Add the missing dependency for the command ums:
- CONFIG_BLK: call of blk_* functions in usb_mass_storage.c
- CONFIG_USB_GADGET: required to select CONFIG_USB_FUNCTION_MASS_STORAGE

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
diff --git a/cmd/Kconfig b/cmd/Kconfig
index f962bb7..a9fb4ee 100644
--- a/cmd/Kconfig
+++ b/cmd/Kconfig
@@ -1346,8 +1346,11 @@
 config CMD_USB_MASS_STORAGE
 	bool "UMS usb mass storage"
 	select USB_FUNCTION_MASS_STORAGE
+	depends on BLK && USB_GADGET
 	help
-	  USB mass storage support
+	  Enables the command "ums" and the USB mass storage support to the
+	  export a block device: U-Boot, the USB device, acts as a simple
+	  external hard drive plugged on the host USB port.
 
 config CMD_PVBLOCK
 	bool "Xen para-virtualized block device"