uboot-imx

tegra: bootcmd: start USB only when needed

Instead of initializing USB as soon as bootcmd is executed, defer it
until the first boot device that (potentially in the case of network)
uses USB is scanned. This avoids initializing USB when booting from MMC,
so speeds that up.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Tom Warren <twarren@nvidia.com>