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/nyan-big_defconfig b/configs/nyan-big_defconfig
index dc965cc..8ce1c2f 100644
--- a/configs/nyan-big_defconfig
+++ b/configs/nyan-big_defconfig
@@ -36,6 +36,7 @@
 CONFIG_DM_USB=y
 CONFIG_USB_GADGET=y
 CONFIG_CI_UDC=y
+CONFIG_USB_GADGET_DOWNLOAD=y
 CONFIG_DM_VIDEO=y
 CONFIG_DISPLAY=y
 CONFIG_VIDEO_TEGRA124=y