kconfig: provide an option for enforcing Kconfig settings

This commit introduces an internal FORCE_CONF_FILE CMake setting which
allows higher order build systems to generate a configuration file
which will always take precedence.

This means that in case a user tries to change any setting to be
different than the defined value in the FORCE_CONF_FILE provide file(s),
then a warning will be printed and the setting will be reset to the
value given in the FORCE_CONF_FILE file.

Example of such warning:
   <path>/.config.sysbuild:1: warning: BOOTLOADER_MCUBOOT
   (defined at Kconfig.zephyr:766) set more than once.
   Old value "n", new value "y".

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>

cmake/modules/kconfig.cmake

@@ -306,10 +306,14 @@ if(EXISTS ${DOTCONFIG} AND EXISTS ${merge_config_files_checksum_file})
 endif()
 
 if(CREATE_NEW_DOTCONFIG)
-  set(input_configs_are_handwritten --handwritten-input-configs)
+  set(input_configs_flags --handwritten-input-configs)
-  set(input_configs ${merge_config_files})
+  set(input_configs ${merge_config_files} ${FORCED_CONF_FILE})
 else()
-  set(input_configs ${DOTCONFIG})
+  set(input_configs ${DOTCONFIG} ${FORCED_CONF_FILE})
+endif()
+
+if(DEFINED FORCED_CONF_FILE)
+  list(APPEND input_configs_flags --forced-input-configs)
 endif()
 
 cmake_path(GET AUTOCONF_H PARENT_PATH autoconf_h_path)
@@ -324,7 +328,7 @@ execute_process(
   ${PYTHON_EXECUTABLE}
   ${ZEPHYR_BASE}/scripts/kconfig/kconfig.py
   --zephyr-base=${ZEPHYR_BASE}
-  ${input_configs_are_handwritten}
+  ${input_configs_flags}
   ${KCONFIG_ROOT}
   ${DOTCONFIG}
   ${AUTOCONF_H}

scripts/kconfig/kconfig.py

@@ -11,6 +11,7 @@
 
 import argparse
 import os
+import re
 import sys
 import textwrap
 
@@ -41,7 +42,16 @@ def main():
         kconf.warn_assign_override = False
         kconf.warn_assign_redun = False
 
-    # Load configuration files
+    if args.forced_input_configs:
+        # Do not warn on a redundant config.
+        # The reason is that a regular .config will be followed by the forced
+        # config which under normal circumstances should be identical to the
+        # configured setting.
+        # Only if user has modified to a value that gets overruled by the forced
+        # a warning shall be issued.
+        kconf.warn_assign_redun = False
+
+    # Load files
     print(kconf.load_config(args.configs_in[0]))
     for config in args.configs_in[1:]:
         # replace=False creates a merged configuration
@@ -76,18 +86,30 @@ def main():
     # fast.
     kconf.write_config(os.devnull)
 
+    warn_only = r"warning:.*set more than once."
+
     if kconf.warnings:
+        if args.forced_input_configs:
+            error_out = False
+        else:
+            error_out = True
+
         # Put a blank line between warnings to make them easier to read
         for warning in kconf.warnings:
             print("\n" + warning, file=sys.stderr)
 
+            if not error_out and not re.search(warn_only, warning):
+                # The warning is not a warn_only, fail the Kconfig.
+                error_out = True
+
         # Turn all warnings into errors, so that e.g. assignments to undefined
         # Kconfig symbols become errors.
         #
         # A warning is generated by this script whenever a symbol gets a
         # different value than the one it was assigned. Keep that one as just a
         # warning for now.
-        err("Aborting due to Kconfig warnings")
+        if error_out:
+            err("Aborting due to Kconfig warnings")
 
     # Write the merged configuration and the C header
     print(kconf.write_config(args.config_out))
@@ -258,6 +280,14 @@ def parse_args():
                              "handwritten fragments and do additional checks "
                              "on them, like no promptless symbols being "
                              "assigned")
+    parser.add_argument("--forced-input-configs",
+                        action="store_true",
+                        help="Indicate the input configuration files are "
+                             "followed by an forced configuration file."
+                             "The forced configuration is used to forcefully "
+                             "set specific configuration settings to a "
+                             "pre-defined value and thereby remove any user "
+                             " adjustments.")
     parser.add_argument("--zephyr-base",
                         help="Path to current Zephyr installation")
     parser.add_argument("kconfig_file",