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/board/samsung/common/Makefile b/board/samsung/common/Makefile
index 93347ef..5fb01ce 100644
--- a/board/samsung/common/Makefile
+++ b/board/samsung/common/Makefile
@@ -6,7 +6,7 @@
 #
 
 obj-$(CONFIG_SOFT_I2C_MULTI_BUS) += multi_i2c.o
-obj-$(CONFIG_USBDOWNLOAD_GADGET) += gadget.o
+obj-$(CONFIG_USB_GADGET_DOWNLOAD) += gadget.o
 obj-$(CONFIG_MISC_COMMON) += misc.o
 
 ifndef CONFIG_SPL_BUILD