Bootrom function _reset_to_usb_boot only works if XOSC is running

[jannic]

The bootrom function _reset_to_usb_boot (code 'U', 'B') fails to reboot to USB if the XOSC is not running.

This seems to be caused by the use of watchdog_reboot(), which switches to this oscillator without checking if it is running: https://github.com/raspberrypi/pico-bootrom/blob/master/usb_device_tiny/runtime.c#L94-L96
(called from https://github.com/raspberrypi/pico-bootrom/blob/master/bootrom/bootrom_main.c#L222)

As a workaround, before calling the bootrom function, the oscillator should be started if it's not running. (It's not necessary to switch to the XOSC oscillator.)
It may also be necessary to ensure other parts of the clock configuration are compatible with the XOSC oscillator. I didn't check that. However, the default configuration after bootup is compatible, it's only missing the XOSC startup.