usb: USB download gadget and functions config options coherent naming

This introduces a coherent scheme for naming USB download gadget and functions
config options. The download USB gadget config option is moved to
CONFIG_USB_GADGET_DOWNLOAD for better consistency with other gadgets and each
function's config option is moved to a CONFIG_USB_FUNCTION_ prefix.

Signed-off-by: Paul Kocialkowski <contact@paulk.fr>
Tested-by: Lukasz Majewski <l.majewski@samsung.com>

Test HW: Odroid_XU3 (Exynos5422), trats (Exynos4210)
diff --git a/drivers/dfu/Makefile b/drivers/dfu/Makefile
index 5cc535e..cebea30 100644
--- a/drivers/dfu/Makefile
+++ b/drivers/dfu/Makefile
@@ -5,7 +5,7 @@
 # SPDX-License-Identifier:	GPL-2.0+
 #
 
-obj-$(CONFIG_DFU_FUNCTION) += dfu.o
+obj-$(CONFIG_USB_FUNCTION_DFU) += dfu.o
 obj-$(CONFIG_DFU_MMC) += dfu_mmc.o
 obj-$(CONFIG_DFU_NAND) += dfu_nand.o
 obj-$(CONFIG_DFU_RAM) += dfu_ram.o