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/colibri_vf_defconfig b/configs/colibri_vf_defconfig
index ccba59b..ba6ee38 100644
--- a/configs/colibri_vf_defconfig
+++ b/configs/colibri_vf_defconfig
@@ -20,3 +20,4 @@
 CONFIG_USB=y
 CONFIG_USB_GADGET=y
 CONFIG_CI_UDC=y
+CONFIG_USB_GADGET_DOWNLOAD=y