systemcmds/reboot: Fix reboot command for kernel mode

[pussuw]

Fixes reboot command by:

• Start user workqueue thread in kernel mode by "lazy init"
• Provide hartid in a0 for __start() as required

[jlaitine]

why not just add something like "target_compile_options(mmymodule.... PRIVATE -D__KERNEL__)" into CMakeLists.txt for the module, which compiles this and goes into kernel? This is fine as well, don't really care :)

[pussuw]

Yes this is the preferred way but in this particular case, the library that takes shutdown.cpp is px4_platform and it is unambiguous where it goes. It is still not obvious / clear to me how that library should be handled. It is still just a massive mess in my head.

[pussuw]

I separated shutdown into its own target, now its compile time flags can be adjusted

[pussuw]

Hmm now that I look at this removing reboot/shutdown from user should be unnecessary. I think the problem is like with mavlink cdcacm checker, i.e. the user work queue is not running in kernel mode:

void cdcacm_init(void)
{
#ifdef CONFIG_BUILD_KERNEL
	// Must start the worker as it is not automatically started by the system
	work_usrstart();
#endif
	work_queue(LPWORK, &usb_serial_work, mavlink_usb_check, nullptr, 0);
}

[pussuw]

What I was wondering is what the shutdown hooks in shutdown.cpp are for. Only reference I can find is this:

px4_register_shutdown_hook(&Logger::request_stop_static);

[jlaitine]

LGTM