Kernel fails to build with `PYOPT=0`

[matejcik]

In the following code, printf is called when PYOPT is set to 0. But printf is not available in kernel context. And including <stdio.h> does not seem to be the way to go :woman_shrugging: https://github.com/trezor/trezor-firmware/blob/matejcik/global-layout-only3/core/embed/kernel/main.c#L51-L69

This does not affect current builds because PYOPT is never 0 in kernel, but it blocks #4234 .

The printf statements are supposed to output to the USB VCP on real device (and to console in emulator, but that's irrelevant because emulator does not use optiga)

[cepetr]

Yes, that seems to be a system problem. Currently, the kernel itself is not aware of usb_vcp, as it is now initialized by the unprivileged app via a syscall. We need to move the USB initialization to the kernel and make usb_vcp available to the kernel as well. The unprivileged app would then call the print syscall instead of directly accessing the usb_vcp interface.