Kconfig: Introduce CONFIG_WERROR

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>