kconfig: Add transitional symbol attribute for migration support

During kernel option migrations (e.g. CONFIG_CFI_CLANG to CONFIG_CFI),
existing .config files need to maintain backward compatibility while
preventing deprecated options from appearing in newly generated
configurations. This is challenging with existing Kconfig mechanisms
because:

1. Simply removing old options breaks existing .config files.
2. Manually listing an option as "deprecated" leaves it needlessly
   visible and still writes them to new .config files.
3. Using any method to remove visibility (.e.g no 'prompt', 'if n',
   etc) prevents the option from being processed at all.

Add a "transitional" attribute that creates symbols which are:
- Processed during configuration (can influence other symbols' defaults)
- Hidden from user menus (no prompts appear)
- Omitted from newly written .config files (gets migrated)
- Restricted to only having help sections (no defaults, selects, etc)
  making it truly just a "prior value pass-through" option.

The transitional syntax requires a type argument and prevents type
redefinition:

    config NEW_OPTION
        bool "New option"
        default OLD_OPTION

    config OLD_OPTION
        bool
        transitional
        help
          Transitional config for OLD_OPTION migration.

This allows seamless migration: olddefconfig processes existing
CONFIG_OLD_OPTION=y settings to enable CONFIG_NEW_OPTION=y, while
CONFIG_OLD_OPTION is omitted from newly generated .config files.

Added positive and negative testing via "testconfig" make target.

Co-developed-by: Vegard Nossum <vegard.nossum@oracle.com>
Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Link: https://lore.kernel.org/r/20250923213422.1105654-2-kees@kernel.org
Signed-off-by: Kees Cook <kees@kernel.org>

scripts/kconfig/tests/transitional/Kconfig

@@ -0,0 +1,100 @@
+# SPDX-License-Identifier: GPL-2.0
+# Test transitional symbols for config migration with all Kconfig types
+
+# Enable module support for tristate testing
+config MODULES
+	bool "Enable loadable module support"
+	modules
+	default y
+
+# Basic migration tests for all types
+config NEW_BOOL
+	bool "New bool option"
+	default OLD_BOOL
+
+config OLD_BOOL
+	bool
+	transitional
+
+config NEW_TRISTATE
+	tristate "New tristate option"
+	default OLD_TRISTATE
+
+config OLD_TRISTATE
+	tristate
+	transitional
+
+config NEW_STRING
+	string "New string option"
+	default OLD_STRING
+
+config OLD_STRING
+	string
+	transitional
+
+config NEW_HEX
+	hex "New hex option"
+	default OLD_HEX
+
+config OLD_HEX
+	hex
+	transitional
+
+config NEW_INT
+	int "New int option"
+	default OLD_INT
+
+config OLD_INT
+	int
+	transitional
+
+# Precedence tests for all types
+config NEW_BOOL_PRECEDENCE
+	bool "New bool option with precedence"
+	default OLD_BOOL_PRECEDENCE
+
+config OLD_BOOL_PRECEDENCE
+	bool
+	transitional
+
+config NEW_STRING_PRECEDENCE
+	string "New string option with precedence"
+	default OLD_STRING_PRECEDENCE
+
+config OLD_STRING_PRECEDENCE
+	string
+	transitional
+
+config NEW_TRISTATE_PRECEDENCE
+	tristate "New tristate option with precedence"
+	default OLD_TRISTATE_PRECEDENCE
+
+config OLD_TRISTATE_PRECEDENCE
+	tristate
+	transitional
+
+config NEW_HEX_PRECEDENCE
+	hex "New hex option with precedence"
+	default OLD_HEX_PRECEDENCE
+
+config OLD_HEX_PRECEDENCE
+	hex
+	transitional
+
+config NEW_INT_PRECEDENCE
+	int "New int option with precedence"
+	default OLD_INT_PRECEDENCE
+
+config OLD_INT_PRECEDENCE
+	int
+	transitional
+
+# Test that help sections are allowed for transitional symbols
+config OLD_WITH_HELP
+	bool
+	transitional
+	help
+	  This transitional symbol has a help section to validate that help is allowed.
+
+config REGULAR_OPTION
+	bool "Regular option"

scripts/kconfig/tests/transitional/__init__.py

@@ -0,0 +1,18 @@
+# SPDX-License-Identifier: GPL-2.0
+"""
+Test transitional symbol migration functionality for all Kconfig types.
+
+This tests that:
+- OLD_* options in existing .config cause NEW_* options to be set
+- OLD_* options are not written to the new .config file
+- NEW_* options appear in the new .config file with correct values
+- All Kconfig types work correctly: bool, tristate, string, hex, int
+- User-set NEW values take precedence over conflicting OLD transitional values
+"""
+
+def test(conf):
+    # Run olddefconfig to process the migration with the initial config
+    assert conf.olddefconfig(dot_config='initial_config') == 0
+
+    # Check that the configuration matches expected output
+    assert conf.config_contains('expected_config')

scripts/kconfig/tests/transitional/expected_config

@@ -0,0 +1,12 @@
+CONFIG_MODULES=y
+CONFIG_NEW_BOOL=y
+CONFIG_NEW_TRISTATE=m
+CONFIG_NEW_STRING="test string"
+CONFIG_NEW_HEX=0x1234
+CONFIG_NEW_INT=42
+# CONFIG_NEW_BOOL_PRECEDENCE is not set
+CONFIG_NEW_STRING_PRECEDENCE="user value"
+CONFIG_NEW_TRISTATE_PRECEDENCE=y
+CONFIG_NEW_HEX_PRECEDENCE=0xABCD
+CONFIG_NEW_INT_PRECEDENCE=100
+# CONFIG_REGULAR_OPTION is not set

scripts/kconfig/tests/transitional/initial_config

@@ -0,0 +1,16 @@
+CONFIG_MODULES=y
+CONFIG_OLD_BOOL=y
+CONFIG_OLD_TRISTATE=m
+CONFIG_OLD_STRING="test string"
+CONFIG_OLD_HEX=0x1234
+CONFIG_OLD_INT=42
+# CONFIG_NEW_BOOL_PRECEDENCE is not set
+CONFIG_OLD_BOOL_PRECEDENCE=y
+CONFIG_NEW_STRING_PRECEDENCE="user value"
+CONFIG_OLD_STRING_PRECEDENCE="old value"
+CONFIG_NEW_TRISTATE_PRECEDENCE=y
+CONFIG_OLD_TRISTATE_PRECEDENCE=m
+CONFIG_NEW_HEX_PRECEDENCE=0xABCD
+CONFIG_OLD_HEX_PRECEDENCE=0x5678
+CONFIG_NEW_INT_PRECEDENCE=100
+CONFIG_OLD_INT_PRECEDENCE=200