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>

util/kconfig/patches/series

@@ -2,7 +2,6 @@
 0002-Kconfig-Change-symbol-override-from-warning-to-notic.patch
 0003-util-kconfig-conf.c-Fix-newline-in-error-printf.patch
 0004-src-util-Use-NULL-instead-of-0-for-pointer.patch
-0005-util-kconfig-Ignore-extra-symbols-in-configs-instead.patch
 0006-util-kconfig-Set-parameter-of-mkdir-to-only-one-for-.patch
 0007-kconfig-Allow-KCONFIG_STRICT-outside-of-confdata.c.patch
 0008-kconfig-Add-wildcard-support-for-source.patch