usb: gadget: Move CONFIG_USB_GADGET_DOWNLOAD to Kconfig

While at it, remove obsolete CONFIG_USBDOWNLOAD_GADGET option from some
config headers. This is also probably fixes am335x_baltos board.

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
diff --git a/drivers/usb/gadget/Kconfig b/drivers/usb/gadget/Kconfig
index 5838d6e..969873c 100644
--- a/drivers/usb/gadget/Kconfig
+++ b/drivers/usb/gadget/Kconfig
@@ -80,4 +80,10 @@
 config USB_GADGET_DUALSPEED
 	bool
 
+config USB_GADGET_DOWNLOAD
+	bool "Enable USB download gadget"
+	help
+	  Composite USB download gadget support (g_dnl) for download functions.
+	  This code works on top of composite gadget.
+
 endif # USB_GADGET