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/include/configs/am43xx_evm.h b/include/configs/am43xx_evm.h
index 8bea4c2..3643489 100644
--- a/include/configs/am43xx_evm.h
+++ b/include/configs/am43xx_evm.h
@@ -125,6 +125,7 @@
 #if defined(CONFIG_SPL_BUILD) && !defined(CONFIG_SPL_USBETH_SUPPORT)
 #undef CONFIG_USB_DWC3
 
+#undef CONFIG_USB_GADGET_DOWNLOAD
 #undef CONFIG_USB_GADGET_VBUS_DRAW
 #undef CONFIG_USB_GADGET_DUALSPEED
 #endif
@@ -135,7 +136,6 @@
 #define CONFIG_USB_DWC3_OMAP
 #define CONFIG_USB_DWC3_GADGET
 
-#define CONFIG_USB_GADGET_DOWNLOAD
 #define CONFIG_G_DNL_MANUFACTURER "Texas Instruments"
 #define CONFIG_G_DNL_VENDOR_NUM 0x0403
 #define CONFIG_G_DNL_PRODUCT_NUM 0xBD00