Fix gcc 10 link issues

- avoid linker error by providing string.[ch] in bootrom

bootrom/string.h

@@ -0,0 +1,25 @@
+//-----------------------------------------------------------------------------
+// Jonathan Westhues, Aug 2005
+// Copyright (C) 2010 Hector Martin "marcan" <marcan@marcansoft.com>
+//
+// This code is licensed to you under the terms of the GNU GPL, version 2 or,
+// at your option, any later version. See the LICENSE.txt file for the text of
+// the license.
+//-----------------------------------------------------------------------------
+// Common string.h functions
+//-----------------------------------------------------------------------------
+
+#ifndef __STRING_H
+#define __STRING_H
+
+#include <stddef.h>
+
+void *memcpy(void *dest, const void *src, size_t len);
+void *memset(void *dest, int c, size_t len);
+void *memmove(void *dest, const void *src, size_t len);
+int memcmp(const void *av, const void *bv, size_t len);
+size_t strlen(const char *str);
+char *strncat(char *dest, const char *src, size_t n);
+char *strcat(char *dest, const char *src);
+
+#endif /* __STRING_H */