tegra: Test on CONFIG_CMD_USB being enabled for distro bootcmd

Reuse the common logic to allow for BOOT_TARGET_DEVICES to list USB as a
possibility if we're building for a platform that will have USB but not
if we don't, so that we don't hit the link-time check for trying to have
USB boot on a non-USB system.

Acked-by: Tom Warren <twarren@nvidia.com>
Signed-off-by: Tom Rini <trini@konsulko.com>