USB: make "usb start" start usb only once

Currently we've this magic in include/config_distro_bootcmd.h to avoid
scanning the usb bus multiple times.

And it does not work when also using an usb keyboard because then the
preboot command has already scanned the bus, so we're still scanning it
twice.

This commit makes "usb start" only start usb if it is no already started,
allowing us to remove all the magic for it from include/config_distro_bootcmd.h
and just call it unconditionally.

This also causes "usb start" and "usb reset" to actually do what their
different names suggest, rather then both of them doing exactly the same.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>