[kernel] Move the logger from kutil into kernel

Part one of a series of code moves. The kutil library is not very
useful, as most of its code is kernel-specific. This was originally for
testing purposes, but that can be achieved in other ways with the
current build system. I find this mostly creates a strange division in
the kernel code.

Instead, I'm going to move everything kernel-specific to actually be in
the kernel, and replace kutil with just 'util' for generic utility code
I want to share.

This commit:

- Moves the logger into the kernel.
- Updates the 'printf' library used from mpaland/printf to
  eyalroz/printf and moved it into the kernel, as it's only used by the
  logger in kutil.
- Removes some other unused kutil headers from some files, to help
  future code rearrangement.

Note that the (now redundant-seeming) log.cpp/h in kernel is currently
still there - these files are more about log output than the logging
system, and will get replaced once I add user-space log output.

other_software.md

@@ -35,14 +35,16 @@ Boost Software license:
 
 ## printf
 
-jsix uses Marco Paland's tiny [printf][] library for its `*printf()` functions,
-which is also released under the terms of the MIT license:
+jsix uses Marco Paland and Eyal Rozenberg's tiny [printf][] library for
+in-kernel `*printf()` functions, which is also released under the terms of the
+MIT license:
 
-[printf]: https://github.com/mpaland/printf
+[printf]: https://github.com/eyalroz/printf
 
 > The MIT License (MIT)
 >
 > Copyright (c) 2014 Marco Paland
+> Copyright (c) 2021 Eyal Rozenberg
 >
 > Permission is hereby granted, free of charge, to any person obtaining a copy
 > of this software and associated documentation files (the "Software"), to deal