A number of small lfs_util.h QOL changes

- Changed the name of the LFS_CONFIG macro to LFS_UTIL to avoid
  confusion with the lfs_config struct. This also hints that LFS_UTIL
  is related to lfs_util.h.

  LFS_UTIL allows the user to override lfs_util.h so they can provide
  their own system-level dependencies such as malloc, tracing, builtins,
  stdint definitions, string.h, and others.

- Removed stdlib includes from lfs.h, these should all go through
  lfs_util.h to let users override these definitions if stdlib is
  unavailable on their system.

- Moved error code definitions to lfs_util.h. This lets users override
  the error codes to replace them with their own error codes and avoid
  a translation layer in some situations. Note the error codes must
  still be in the range of a negative int.

- Used proper stdint definitions in lfs_scmp.

lfs_util.c

@@ -7,7 +7,7 @@
 #include "lfs_util.h"
 
 // Only compile if user does not provide custom config
-#ifndef LFS_CONFIG
+#ifndef LFS_UTIL
 
 
 // Software CRC implementation with small lookup table