dbg_printf for kernel debugging

[cepetr]

This tiny pull request introduces the dbg_printf routine, which can be used inside the kernel for temporary debugging purposes.

Currently, dbg_printf() outputs all characters to the SWO output, which can be displayed in an IDE or a standalone application like STM32CubeProg while the device is connected to an ST-Link.

In the future, we might extend dbg_printf() to allow switching between SWO, USB VCP, and other outputs, or disabling it in release builds to make it more versatile.

[github-actions]

core UI changes	device test	click test	persistence test
T2T1 Model T	test(screens) main(screens)	test(screens) main(screens)	test(screens) main(screens)
T3B1 Safe 3	test(screens) main(screens)	test(screens) main(screens)	test(screens) main(screens)
T3T1 Safe 5	test(screens) main(screens)	test(screens) main(screens)	test(screens) main(screens)
All	main(screens)