Makefile: add kselftest-merge

Add kselftest-merge to enable the dependencies of kernel config
for all the kselftest testcases. User should call it after create
the kernel configuration(.config), eg:

$ make kselftest-merge

Signed-off-by: Bamvor Jian Zhang <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

Makefile

···
 kselftest-clean:
 	$(Q)$(MAKE) -C tools/testing/selftests clean
 
+PHONY += kselftest-merge
+kselftest-merge:
+	$(if $(wildcard $(objtree)/.config),, $(error No .config exists, config your kernel first!))
+	$(Q)$(CONFIG_SHELL) $(srctree)/scripts/kconfig/merge_config.sh \
+		-m $(objtree)/.config \
+		$(srctree)/tools/testing/selftests/*/config
+	+$(Q)$(MAKE) -f $(srctree)/Makefile olddefconfig
+
 # ---------------------------------------------------------------------------
 # Modules
 
···
 	@echo  '                    Build, install, and boot kernel before'
 	@echo  '                    running kselftest on it'
 	@echo  '  kselftest-clean - Remove all generated kselftest files'
+	@echo  '  kselftest-merge - Merge all the config dependencies of kselftest to existed'
+	@echo  '                    .config.'
 	@echo  ''
 	@echo  'Kernel packaging:'
 	@$(MAKE) $(build)=$(package-dir) help