platform, common, lowlevel modules cleanup

[cepetr]

This PR focuses on cleaning up tangled code in the platform.c/h, common.c/h, lowlevel.c/h modules, as well as the Unix emulator.

1. Removed some inappropriate dependencies (such as CPU clock setting and the main_clean_exit cyclic dependency) from the emulator code.
2. Most code from the lowlevel module has been relocated to the new option_bytes module.
3. Introduced a small pvd (Power Voltage Detector) module, which was previously split across two different places.
4. Introduced a small reset_flags module, which was previously located in the lowlevel module. Although it's currently not very useful, it might be improved in the future.
5. Most code from platform.c has been moved to a new startup_init module, which better reflects its purpose.
6. The platform.h header no longer exists, freeing up the name for potential future use.

Possible next steps:

1. common.h now only contains just utility functions. We might consider renaming this module to better reflect its purpose.
2. The routines in utils.s seem slightly out of place and could be better organized between bootutil and startup_init.

[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)	2724
T3T1	test(screens) main(screens)	test(screens) main(screens)	test(screens) main(screens)
All	main(screens)