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/am437x_gp_evm_defconfig b/configs/am437x_gp_evm_defconfig
index e628bd4..b02f8ad 100644
--- a/configs/am437x_gp_evm_defconfig
+++ b/configs/am437x_gp_evm_defconfig
@@ -23,3 +23,4 @@
 CONFIG_USB=y
 CONFIG_USB_DWC3=y
 CONFIG_USB_GADGET=y
+CONFIG_USB_GADGET_DOWNLOAD=y