Kconfig: Introduce CONFIG_WERROR

Add a new config option under "General setup" to enable the -Werror flag
when building U-Boot. This is useful during development to help catch
mistakes.

This is based on a similar config option added to the Linux kernel by
Linus in 2021 - see Linux commit 3fe617ccafd6 ("Enable '-Werror' by
default for all kernel builds"). The modification of KBUILD_CFLAGS is
done in Makefile.extrawarn, matching where it was moved in the kernel by
Linux commit e88ca24319e4 ("kbuild: consolidate warning flags in
scripts/Makefile.extrawarn").

Signed-off-by: Paul Barker <paul.barker.ct@bp.renesas.com>
diff --git a/Kconfig b/Kconfig
index 6379a45..70dc385 100644
--- a/Kconfig
+++ b/Kconfig
@@ -27,6 +27,17 @@
 	  code that relies on deprecated features that will be removed and
 	  the conversion deadline has passed.
 
+config WERROR
+	bool "Compile U-Boot with warnings as errors"
+	help
+	  A U-Boot build should not cause any compiler warnings, and this
+	  enables the '-Werror' flag to enforce that rule.
+
+	  However, if you have a new (or very old) compiler or linker with odd
+	  and unusual warnings, or you have some architecture with problems,
+	  you may need to disable this config option in order to
+	  successfully build U-Boot.
+
 config LOCALVERSION
 	string "Local version - append to U-Boot release"
 	help