Return a message from Kconfig.write_autoconf()

Like for Kconfig.write_config() and Kconfig.write_min_config(), return a
string from Kconfig.write_autoconf() with a message saying that the
header got saved, or that there were no changes to it. Can be handy in
tools.

Also make the "no change" message for the various files more specific,
by mentioning what type of file it is (configuration, header, etc.)

Return True/False from Kconfig._write_if_changed() to indicate if the
file was updated. This also allows it to be reused in
Kconfig.write_min_config().

kconfiglib.py

@@ -1444,12 +1444,18 @@ def write_autoconf(self, filename=None, header=None):
           KCONFIG_AUTOHEADER_HEADER had when the Kconfig instance was created
           will be used if it was set, and no header otherwise. See the
           Kconfig.header_header attribute.
+
+        Returns a string with a message saying that the header got saved, or
+        that there were no changes to it. This is meant to reduce boilerplate
+        in tools, which can do e.g. print(kconf.write_autoconf()).
         """
         if filename is None:
             filename = os.getenv("KCONFIG_AUTOHEADER",
                                  "include/generated/autoconf.h")
 
-        self._write_if_changed(filename, self._autoconf_contents(header))
+        if self._write_if_changed(filename, self._autoconf_contents(header)):
+            return "Kconfig header saved to '{}'".format(filename)
+        return "No change to Kconfig header in '{}'".format(filename)
 
     def _autoconf_contents(self, header):
         # write_autoconf() helper. Returns the contents to write as a string,
@@ -1564,7 +1570,7 @@ def write_config(self, filename=None, header=None, save_old=True,
 
         contents = self._config_contents(header)
         if self._contents_eq(filename, contents):
-            return "No change to '{}'".format(filename)
+            return "No change to configuration in '{}'".format(filename)
 
         if save_old:
             _save_old(filename)
@@ -1677,18 +1683,14 @@ def write_min_config(self, filename, header=None):
           be used if it was set, and no header otherwise. See the
           Kconfig.config_header attribute.
 
-        Returns a string with a message saying which file got saved. This is
-        meant to reduce boilerplate in tools, which can do e.g.
+        Returns a string with a message saying the minimal configuration got
+        saved, or that there were no changes to it. This is meant to reduce
+        boilerplate in tools, which can do e.g.
         print(kconf.write_min_config()).
         """
-        contents = self._min_config_contents(header)
-        if self._contents_eq(filename, contents):
-            return "No change to '{}'".format(filename)
-
-        with self._open(filename, "w") as f:
-            f.write(contents)
-
-        return "Minimal configuration saved to '{}'".format(filename)
+        if self._write_if_changed(filename, self._min_config_contents(header)):
+            return "Minimal configuration saved to '{}'".format(filename)
+        return "No change to minimal configuration in '{}'".format(filename)
 
     def _min_config_contents(self, header):
         # write_min_config() helper. Returns the contents to write as a string,
@@ -2264,10 +2266,15 @@ def _write_if_changed(self, filename, contents):
         # differs, but it breaks stuff like write_config("/dev/null"), which is
         # used out there to force evaluation-related warnings to be generated.
         # This simple version is pretty failsafe and portable.
+        #
+        # Returns True if the file has changed and is updated, and False
+        # otherwise.
 
-        if not self._contents_eq(filename, contents):
-            with self._open(filename, "w") as f:
-                f.write(contents)
+        if self._contents_eq(filename, contents):
+            return False
+        with self._open(filename, "w") as f:
+            f.write(contents)
+        return True
 
     def _contents_eq(self, filename, contents):
         # Returns True if the contents of 'filename' is 'contents' (a string),