kbuild: unify W=e and CONFIG_WERROR

The two mechanisms have the same effect, unify their implementation.

Also avoid spurious rebuilds when switching between the two.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Reviewed-by: Nicolas Schier <nsc@kernel.org>
Link: https://lore.kernel.org/r/20250814-kbuild-werror-v2-2-c01e596309d2@linutronix.de
Signed-off-by: Nathan Chancellor <nathan@kernel.org>

Author: t-8ch

scripts/Makefile.extrawarn

@@ -25,8 +25,6 @@ ifneq ($(CONFIG_FRAME_WARN),0)
 KBUILD_CFLAGS += -Wframe-larger-than=$(CONFIG_FRAME_WARN)
 endif
 
-KBUILD_CPPFLAGS-$(CONFIG_WERROR) += -Werror
-KBUILD_CPPFLAGS += $(KBUILD_CPPFLAGS-y)
 KBUILD_CFLAGS-$(CONFIG_CC_NO_ARRAY_BOUNDS) += -Wno-array-bounds
 
 ifdef CONFIG_CC_IS_CLANG
@@ -214,9 +212,9 @@ KBUILD_CFLAGS += -Wno-unused-parameter
 endif
 
 #
-# W=e - error out on warnings
+# W=e and CONFIG_WERROR - error out on warnings
 #
-ifneq ($(findstring e, $(KBUILD_EXTRA_WARN)),)
+ifneq ($(findstring e, $(KBUILD_EXTRA_WARN))$(CONFIG_WERROR),)
 
 KBUILD_CPPFLAGS		+= -Werror