treewide: Test on CONFIG_USB_HOST (or CONFIG_CMD_USB) not CONFIG_USB

As the logic here is only used when we have a USB host controller, test
on CONFIG_USB_HOST rather than CONFIG_USB in general.  This lets us move
towards using CONFIG_USB only as a menu symbol to say that we have some
form of USB, and then USB_HOST or USB_GADGET depending on the role that
USB plays within the build.

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/cmd/Kconfig b/cmd/Kconfig
index f196e6c..e40d390 100644
--- a/cmd/Kconfig
+++ b/cmd/Kconfig
@@ -1322,7 +1322,7 @@
 
 config CMD_USB
 	bool "usb"
-	depends on USB
+	depends on USB_HOST
 	select HAVE_BLOCK_DEVICE
 	help
 	  USB support.