libbpf: Allow specifying both ELF and raw BTF for CO-RE BTF override

Use generalized BTF parsing logic, making it possible to parse BTF both from
ELF file, as well as a raw BTF dump. This makes it easier to write custom
tests with manually generated BTFs.
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20201008001025.292064-4-andrii@kernel.org

tools/lib/bpf/libbpf.c

@@ -5842,7 +5842,7 @@ bpf_object__relocate_core(struct bpf_object *obj, const char *targ_btf_path)
 		return 0;
 
 	if (targ_btf_path)
-		targ_btf = btf__parse_elf(targ_btf_path, NULL);
+		targ_btf = btf__parse(targ_btf_path, NULL);
 	else
 		targ_btf = obj->btf_vmlinux;
 	if (IS_ERR_OR_NULL(targ_btf)) {