kbuild: Let builtin have precedence over modules for kselftest-merge

The kselftest-merge target walks all kselftests configs, and merges
them. However, builtin does not have precedence over modules. This
breaks some of the tests, e.g.:

$ grep CONFIG_NF_NAT tools/testing/selftests/{bpf,net}/config
tools/testing/selftests/bpf/config:CONFIG_NF_NAT=y
tools/testing/selftests/net/config:CONFIG_NF_NAT=m

Here, the net config will set NF_NAT to module, which makes it clunky
to run the BPF tests.

Add '-y' to scripts/kconfig/merge_config.sh.
Signed-off-by: Björn Töpel <bjorn@rivosinc.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

Makefile

@@ -1368,7 +1368,7 @@ PHONY += kselftest-merge
 kselftest-merge:
 	$(if $(wildcard $(objtree)/.config),, $(error No .config exists, config your kernel first!))
 	$(Q)find $(srctree)/tools/testing/selftests -name config | \
-		xargs $(srctree)/scripts/kconfig/merge_config.sh -m $(objtree)/.config
+		xargs $(srctree)/scripts/kconfig/merge_config.sh -y -m $(objtree)/.config
 	$(Q)$(MAKE) -f $(srctree)/Makefile olddefconfig
 
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