kbuild: teach kselftest-merge to find nested config files

Current implementation of kselftest-merge only finds config files that
are one level deep using `$(srctree)/tools/testing/selftests/*/config`.

Often, config files are added in nested directories, and do not get
picked up by kselftest-merge.

Use `find` to catch all config files under
`$(srctree)/tools/testing/selftests` instead.

Signed-off-by: Dan Rue <[e-mail]>
Signed-off-by: Masahiro Yamada <[e-mail]>

Makefile

···
 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)find $(srctree)/tools/testing/selftests -name config | \
+		xargs $(srctree)/scripts/kconfig/merge_config.sh -m $(objtree)/.config
 	+$(Q)$(MAKE) -f $(srctree)/Makefile olddefconfig
 
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