J721e does not support USB Host MSC boot, but only supports DFU boot.
Since BOOT_DEVICE_USB is often used for host boot mode and
BOOT_DEVICE_DFU is used for DFU boot, rename BOOT_DEVICE_USB macro to
BOOT_DEVICE_DFU
Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>