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/configs/trats2_defconfig b/configs/trats2_defconfig
index 60fbab9..d7f8405 100644
--- a/configs/trats2_defconfig
+++ b/configs/trats2_defconfig
@@ -17,3 +17,4 @@
 CONFIG_DM_USB=y
 CONFIG_USB_GADGET=y
 CONFIG_USB_GADGET_DWC2_OTG=y
+CONFIG_USB_GADGET_DOWNLOAD=y