usb: Fastboot function config for better consistency with other functions

USB download gadget functions such as thor and dfu have a separate config option
for the USB gadget part of the code, independent from the command part.
This switches the fastboot USB gadget to the same scheme, for better
consistency.

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/README b/README
index c13a383..1e04cf8 100644
--- a/README
+++ b/README
@@ -1716,6 +1716,9 @@
 		sending again an USB request to the device.
 
 - USB Device Android Fastboot support:
+		CONFIG_USB_FUNCTION_FASTBOOT
+		This enables the USB part of the fastboot gadget
+
 		CONFIG_CMD_FASTBOOT
 		This enables the command "fastboot" which enables the Android
 		fastboot mode for the platform's USB device. Fastboot is a USB