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/doc/README.android-fastboot b/doc/README.android-fastboot
index 04411e9..c2a2418 100644
--- a/doc/README.android-fastboot
+++ b/doc/README.android-fastboot
@@ -33,7 +33,7 @@
 The fastboot gadget relies on the USB download gadget, so the following
 options must be configured:
 
-CONFIG_USBDOWNLOAD_GADGET
+CONFIG_USB_GADGET_DOWNLOAD
 CONFIG_G_DNL_VENDOR_NUM
 CONFIG_G_DNL_PRODUCT_NUM
 CONFIG_G_DNL_MANUFACTURER