samples: usb: Make USB console samples call usb_enable()

usb_enable() must be called once by the application.
The application may want to register usb_dc_status_callback
and trace usb status codes (usb_dc_status_code).

After this patch all pre APPLICATION messages will be dropped
as USB console device is enabled in the application.

Application waits for console device until its ready by checking DTR
flag - uart_line_ctrl_get(). This function could be dropped but then
some log messages that were generated before USB device is ready
could also be dropped.

Signed-off-by: Emil Obalski <emil.obalski@nordicsemi.no>