usb: CONFIG_USB_FASTBOOT prefix replacement for consistency

FASTBOOT is defined both by CONFIG_USB_FUNCTION_FASTBOOT AND CONFIG_CMD_FASTBOOT, so it doesn't
make much sense to have a CONFIG_USB_FASTBOOT prefix for fastboot-specific options, especially
given that other config options for fastboot use the CONFIG_FASTBOOT prefix.

This replaces the CONFIG_USB_FASTBOOT prefix with CONFIG_FASTBOOT, for consistency.

Signed-off-by: Paul Kocialkowski <contact@paulk.fr>
diff --git a/README b/README
index 1e04cf8..4e0ff9f 100644
--- a/README
+++ b/README
@@ -1730,12 +1730,12 @@
 		This enables support for booting images which use the Android
 		image format header.
 
-		CONFIG_USB_FASTBOOT_BUF_ADDR
+		CONFIG_FASTBOOT_BUF_ADDR
 		The fastboot protocol requires a large memory buffer for
 		downloads. Define this to the starting RAM address to use for
 		downloaded images.
 
-		CONFIG_USB_FASTBOOT_BUF_SIZE
+		CONFIG_FASTBOOT_BUF_SIZE
 		The fastboot protocol requires a large memory buffer for
 		downloads. This buffer should be as large as possible for a
 		platform. Define this to the size available RAM for fastboot.