[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.

src/kernel/kernel.module

@@ -26,6 +26,7 @@ kernel = module("kernel",
         "interrupts.s",
         "io.cpp",
         "log.cpp",
+        "logger.cpp",
         "memory_bootstrap.cpp",
         "msr.cpp",
         "objects/channel.cpp",
@@ -38,6 +39,7 @@ kernel = module("kernel",
         "page_table.cpp",
         "page_tree.cpp",
         "pci.cpp",
+        "printf/printf.c",
         "scheduler.cpp",
         "serial.cpp",
         "syscalls/channel.cpp",