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/doc/README.android-fastboot b/doc/README.android-fastboot
index 92f2897..ce12bc5 100644
--- a/doc/README.android-fastboot
+++ b/doc/README.android-fastboot
@@ -47,8 +47,8 @@
 
 The fastboot protocol requires a large memory buffer for downloads. This
 buffer should be as large as possible for a platform. The location of the
-buffer and size are set with CONFIG_USB_FASTBOOT_BUF_ADDR and
-CONFIG_USB_FASTBOOT_BUF_SIZE.
+buffer and size are set with CONFIG_FASTBOOT_BUF_ADDR and
+CONFIG_FASTBOOT_BUF_SIZE.
 
 Fastboot partition aliases can also be defined for devices where GPT
 limitations prevent user-friendly partition names such as "boot", "system"