selftests/bpf: Allow selftests to build with older xxd

Currently selftests require xxd with the "-n <name>" option
which allows the user to specify a name not derived from
the input object path.  Instead of relying on this newer
feature, older xxd can be used if we link our desired name
("test_progs_verification_cert") to the input object.

Many distros ship xxd in vim-common package and do not have
the latest xxd with -n support.

Fixes: b720903e2b ("selftests/bpf: Enable signature verification for some lskel tests")
Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
Link: https://lore.kernel.org/r/20251120084754.640405-3-alan.maguire@oracle.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/.gitignore

@@ -23,6 +23,7 @@ test_tcpnotify_user
 test_libbpf
 xdping
 test_cpp
+test_progs_verification_cert
 *.d
 *.subskel.h
 *.skel.h

tools/testing/selftests/bpf/Makefile

@@ -731,7 +731,8 @@ $(VERIFICATION_CERT) $(PRIVATE_KEY): $(VERIFY_SIG_SETUP)
 	$(Q)$(VERIFY_SIG_SETUP) genkey $(BUILD_DIR)
 
 $(VERIFY_SIG_HDR): $(VERIFICATION_CERT)
-	$(Q)xxd -i -n test_progs_verification_cert $< > $@
+	$(Q)ln -fs $< test_progs_verification_cert && \
+	xxd -i test_progs_verification_cert > $@
 
 # Define test_progs test runner.
 TRUNNER_TESTS_DIR := prog_tests
@@ -905,7 +906,8 @@ EXTRA_CLEAN := $(SCRATCH_DIR) $(HOST_SCRATCH_DIR)			\
 	$(addprefix $(OUTPUT)/,*.o *.d *.skel.h *.lskel.h *.subskel.h	\
 			       no_alu32 cpuv4 bpf_gcc			\
 			       liburandom_read.so)			\
-	$(OUTPUT)/FEATURE-DUMP.selftests
+	$(OUTPUT)/FEATURE-DUMP.selftests				\
+	test_progs_verification_cert
 
 .PHONY: docs docs-clean