kbuild: support W=e to make build abort in case of warning

When developing new code/feature, CONFIG_WERROR is most
often turned off, especially for people using make W=12 to
get more warnings.

In such case, turning on -Werror temporarily would require
switching on CONFIG_WERROR in the configuration, building,
then switching off CONFIG_WERROR.

For this use case, this patch introduces a new 'e' modifier
to W= as a short hand for KCFLAGS+=-Werror" so that -Werror
got added to the kernel (built-in) and modules' CFLAGS.
Signed-off-by: Yann Droneaud <ydroneaud@opteya.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

Makefile

@@ -1650,6 +1650,7 @@ help:
 	@echo  '		1: warnings which may be relevant and do not occur too often'
 	@echo  '		2: warnings which occur quite often but may still be relevant'
 	@echo  '		3: more obscure warnings, can most likely be ignored'
+	@echo  '		e: warnings are being treated as errors'
 	@echo  '		Multiple levels can be combined with W=12 or W=123'
 	@echo  ''
 	@echo  'Execute "make" or "make all" to build all targets marked with [*] '

scripts/Makefile.extrawarn

@@ -2,8 +2,8 @@
 # ==========================================================================
 # make W=... settings
 #
-# There are three warning groups enabled by W=1, W=2, W=3.
-# They are independent, and can be combined like W=12 or W=123.
+# There are four warning groups enabled by W=1, W=2, W=3, and W=e
+# They are independent, and can be combined like W=12 or W=123e.
 # ==========================================================================
 
 KBUILD_CFLAGS += $(call cc-disable-warning, packed-not-aligned)
@@ -94,3 +94,12 @@ KBUILD_CFLAGS += $(call cc-option, -Wpacked-bitfield-compat)
 KBUILD_CPPFLAGS += -DKBUILD_EXTRA_WARN3
 
 endif
+
+#
+# W=e - error out on warnings
+#
+ifneq ($(findstring e, $(KBUILD_EXTRA_WARN)),)
+
+KBUILD_CFLAGS += -Werror
+
+endif