RFC 2132, Section 9.5 does not strictly state Bootfile name (option 67) is null
terminated string. So force to append null terminated character at the end of string.
git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@10635 6f19259b-4bc3-4df7-8a09-765794883524