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/zynq_zc702_defconfig b/configs/zynq_zc702_defconfig
index e561784..81d211f 100644
--- a/configs/zynq_zc702_defconfig
+++ b/configs/zynq_zc702_defconfig
@@ -29,3 +29,4 @@
 CONFIG_USB_ULPI=y
 CONFIG_USB_GADGET=y
 CONFIG_CI_UDC=y
+CONFIG_USB_GADGET_DOWNLOAD=y