util/kconfig: Uprev to Linux 6.6's kconfig

Upstream reimplemented KCONFIG_STRICT, just calling it KCONFIG_WERROR.
Therefore, adapt our build system and documentation. Upstream is less
strict at this time, but there's a proposed patch that got imported.

TEST=`util/abuild/abuild -C` output (config.h and
config.build) remains the same. Also, the failure type fixed in
https://review.coreboot.org/c/coreboot/+/11272 can be detected,
which I tested by manually breaking our Kconfig in a similar way.

Change-Id: I322fb08a2f7308b93cff71a5dd4136f1a998773b
Signed-off-by: Patrick Georgi <patrick@coreboot.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/79259
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Martin L Roth <gaumless@gmail.com>
Reviewed-by: Felix Singer <service+coreboot-gerrit@felixsinger.de>

Documentation/getting_started/kconfig.md

@@ -69,9 +69,6 @@ These variables are typically set in the makefiles or on the make command line.
 These variables were added to Kconfig specifically for coreboot and are not
 included in the Linux version.
 
-- KCONFIG_STRICT=value. Define to enable warnings as errors.   This is enabled
-  in coreboot, and should not be changed.
-
 - KCONFIG_NEGATIVES=value. Define to show negative values in the autoconf.h file
   (build/config.h). This is enabled in coreboot, and should not be changed.
 
@@ -102,6 +99,9 @@ included in the Linux version.
 - KCONFIG_SPLITCONFIG=”directory name for individual SYMBOL.h files”.
   coreboot sets this to $(obj)/config.
 
+- KCONFIG_WERROR=value. Define to enable warnings as errors. This is enabled
+  in coreboot, and should not be changed.
+
 #### Used only for ‘make menuconfig’
 - MENUCONFIG_MODE=single_menu.  Set to "single_menu" to enable.  All other
   values disable the option.  This makes submenus appear below the menu option
@@ -1160,10 +1160,6 @@ saved .config file. As always, a 'select' statement overrides any specified
 - coreboot has added the glob operator '*' for the 'source' keyword.
 - coreboot’s Kconfig always defines variables except for strings. In other
   Kconfig implementations, bools set to false/0/no are not defined.
-- coreboot’s version of Kconfig adds the KCONFIG_STRICT environment variable to
-  error out if there are any issues in the Kconfig files.  In the Linux kernel,
-  Kconfig will generate a warning, but will still output an updated .config or
-  config.h file.
 
 
 ## Kconfig Editor Highlighting