During execution of `boot.py` the USB device is not yet initialized. Any
attempt to write to the CDC (eg calling `print()`) would lock up the
device. This commit skips writing when the USB device is not initialized.
Any output from `boot.py` is lost, but the device does not lock up.
Also removed unnecessary declaration of `tusb_init()`.
Signed-off-by: robert-hh <robert@hammelrath.com>