cmd: Change the dependencies between CMD_BIND and USB_GADGET

cmd: Change the dependencies between CMD_BIND and USB_GADGET

Today CMD_BIND defaults to 'y' when USB_ETHER is enabled. In practice,
CMD_BIND should default to 'y' when any USB gadget is enabled not only
USB_ETHER. Let's invert the logic of the dependency and use the weak
'imply' keyword to enforce this.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Tested-by: Mattijs Korpershoek <mkorpershoek@baylibre.com> # on vim3
Link: https://lore.kernel.org/r/20231010090304.49335-2-miquel.raynal@bootlin.com
Signed-off-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>