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

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