Sync patches r14373 and r14375 from main trunk.
1. Pop up error message when user tries to enroll an invalid certificate file into KEK/DB/DBX.
2. Fix a bug that 'SecureBoot' varaible will be updated to NV+AT attribute incorrectly.

git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/branches/UDK2010.SR1@14376 6f19259b-4bc3-4df7-8a09-765794883524