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>