usb: remove Kconfig option CONFIG_USB

The USB configuration option is actually a global switch
to enable USB drivers in general, but currently only
the device controller drivers are meant.

USB device controller drivers also have USB_DEVICE_DRIVER option.
Thus the option USB is actually redundant and can be replaced
by the self-explanatory option USB_DEVICE_DRIVER.
The name USB itself is not unique and should not be used as an
configuration option.

With these changes the option USB_DEVICE_DRIVER generally
enables USB device controller drivers. The option USB_DEVICE_STACK
enables USB device support. It is sufficient to enable only option
USB_DEVICE_STACK because it selects USB_DEVICE_DRIVER.

CONFIG_USB Kconfig option is temporary added to subsys/usb/Kconfig.
This is necessary to pass CI and will be removed again
when the USB configuration has been adapted in modules.

Signed-off-by: Johann Fischer <johann.fischer@nordicsemi.no>