[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.

configs/base.yaml

@@ -26,7 +26,8 @@ variables:
     "-Wpointer-arith", "-Wno-attributes", "-Wno-sign-compare", "-Wno-multichar",
     "-Wno-div-by-zero", "-Wno-endif-labels", "-Wno-pragmas", "-Wno-format-extra-args",
     "-Wno-unused-result", "-Wno-deprecated-declarations", "-Wno-unused-function",
-    "-Wno-address-of-packed-member", "-Wno-invalid-offsetof", "-Werror" ]
+    "-Wno-address-of-packed-member", "-Wno-invalid-offsetof", "-Wno-format-nonliteral",
+    "-Werror" ]
 
   asflags: [
     "-DVERSION_MAJOR=${version_major}",