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/include/configs/socfpga_common.h b/include/configs/socfpga_common.h
index 4c3366a..e8473b8 100644
--- a/include/configs/socfpga_common.h
+++ b/include/configs/socfpga_common.h
@@ -243,10 +243,10 @@
 #define CONFIG_USB_GADGET_VBUS_DRAW	2
 
 /* USB Composite download gadget - g_dnl */
-#define CONFIG_USBDOWNLOAD_GADGET
-#define CONFIG_USB_GADGET_MASS_STORAGE
+#define CONFIG_USB_GADGET_DOWNLOAD
+#define CONFIG_USB_FUNCTION_MASS_STORAGE
 
-#define CONFIG_DFU_FUNCTION
+#define CONFIG_USB_FUNCTION_DFU
 #define CONFIG_DFU_MMC
 #define CONFIG_SYS_DFU_DATA_BUF_SIZE	(32 * 1024 * 1024)
 #define DFU_DEFAULT_POLL_TIMEOUT	300