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/apalis_t30_defconfig b/configs/apalis_t30_defconfig
index a8b4621..882a21e 100644
--- a/configs/apalis_t30_defconfig
+++ b/configs/apalis_t30_defconfig
@@ -19,4 +19,5 @@
 CONFIG_DM_USB=y
 CONFIG_USB_GADGET=y
 CONFIG_CI_UDC=y
+CONFIG_USB_GADGET_DOWNLOAD=y
 CONFIG_USE_PRIVATE_LIBGCC=y