kbuild: fix W= option checks for extra DTC warnings

Kbuild supports 3 levels of extra warnings, and multiple levels can
be combined, like W=12, W=123.  It was added by commit a6de553d
("kbuild: Allow to combine multiple W= levels").

From the log of commit 8654cb8d ("dtc: update warning settings
for new bus and node/property name checks"), I assume:

 - unit_address_vs_reg, simple_bus_reg, etc. belong to level 1
 - node_name_chars_strict, property_name_chars_strict belong to level 2

However, the level 1 warnings are displayed by any argument to W=.
On the other hand, the level 2 warnings are displayed by W=2, but
not by W=12, or W=123.

Use $(findstring ...) like scripts/Makefile.extrawarn.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>

scripts/Makefile.lib

@@ -264,7 +264,7 @@ cmd_gzip = (cat $(filter-out FORCE,$^) | gzip -n -f -9 > $@) || \
 DTC ?= $(objtree)/scripts/dtc/dtc
 
 # Disable noisy checks by default
-ifeq ($(KBUILD_ENABLE_EXTRA_GCC_CHECKS),)
+ifeq ($(findstring 1,$(KBUILD_ENABLE_EXTRA_GCC_CHECKS)),)
 DTC_FLAGS += -Wno-unit_address_vs_reg \
 	-Wno-simple_bus_reg \
 	-Wno-unit_address_format \
@@ -273,7 +273,7 @@ DTC_FLAGS += -Wno-unit_address_vs_reg \
 	-Wno-pci_device_reg
 endif
 
-ifeq ($(KBUILD_ENABLE_EXTRA_GCC_CHECKS),2)
+ifneq ($(findstring 2,$(KBUILD_ENABLE_EXTRA_GCC_CHECKS)),)
 DTC_FLAGS += -Wnode_name_chars_strict \
 	-Wproperty_name_chars_strict
 endif