distro bootcmd: make net boot only optionally start USB

distro bootcmd: make net boot only optionally start USB

Currently, the distro boot commands always enumerate USB devices before
performing network operations. However, depending on the board and end-
user configuration, network devices may not be attached to USB, and so
enumerating USB may not be necessary. Enhance the scripts to make this
step optional, so that the user can decrease boot time if they don't
need USB.

This change is performed by moving the "usb start" invocation into a
standalone variable. If the user desires, they can replace that
variable's value with some no-op command such as "true" instead.

Booting from a USB storage device always needs to enumerate USB devices,
so this action is still hard-coded.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Simon Glass <sjg@chromium.org>