use old nvmutil for now, in lbmk

i'm trying to make nvmutil work on openbsd. the new code
in lbutils is a bit buggy, likely somewhere in mkhtemp.
i'm still debugging it.

Signed-off-by: Leah Rowe <leah@libreboot.org>

util/nvmutil/lib/string.c

@@ -0,0 +1,75 @@
+/* SPDX-License-Identifier: MIT
+ * Copyright (c) 2026 Leah Rowe <leah@libreboot.org>
+ *
+ * String functions
+ */
+
+#include <sys/types.h>
+#include <sys/stat.h>
+
+#include <errno.h>
+#include <stddef.h>
+#include <unistd.h>
+
+#include "../include/common.h"
+
+/* Portable strncmp() that blocks
+ * NULL/empty/unterminated strings
+ */
+
+int
+xstrxcmp(const char *a, const char *b, unsigned long maxlen)
+{
+	unsigned long i;
+
+	if (a == NULL || b == NULL)
+		err(EINVAL, "NULL input to xstrxcmp");
+
+	if (*a == '\0' || *b == '\0')
+		err(EINVAL, "Empty string in xstrxcmp");
+
+	for (i = 0; i < maxlen; i++) {
+
+		unsigned char ac = (unsigned char)a[i];
+		unsigned char bc = (unsigned char)b[i];
+
+		if (ac == '\0' || bc == '\0') {
+			if (ac == bc)
+				return 0;
+			return ac - bc;
+		}
+
+		if (ac != bc)
+			return ac - bc;
+	}
+
+	err(EINVAL, "Unterminated string in xstrxcmp");
+
+	errno = EINVAL;
+	return -1;
+}
+
+/* Portable strncmp() that blocks
+ * NULL/empty/unterminated strings
+ */
+
+unsigned long
+xstrxlen(const char *scmp, unsigned long maxlen)
+{
+	unsigned long xstr_index;
+
+	if (scmp == NULL)
+		err(EINVAL, "NULL input to xstrxlen");
+
+	if (*scmp == '\0')
+		err(EINVAL, "Empty string in xstrxlen");
+
+	for (xstr_index = 0;
+	    xstr_index < maxlen && scmp[xstr_index] != '\0';
+	    xstr_index++);
+
+	if (xstr_index == maxlen)
+		err(EINVAL, "Unterminated string in xstrxlen");
+
+	return xstr_index;
+}