1. Update the parsing logic of DHCP message in PXE driver.

2. Append null terminated character at the end of option 67.
Signed-off-by: Fu Siyuan <siyuan.fu@intel.com>
Reviewed-by: Dong, Guo <guo.dong@intel.com>
Reviewed-by: Jin, Eric <eric.jin@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@15099 6f19259b-4bc3-4df7-8a09-765794883524