Add tboot (trusted boot) support to gummiboot

This pull request:
- adds trusted boot support to gummiboot via the tboot module and Intel's Trusted Execution Technology (TXT)
- allows booting non efi_stub kernels 
- adds multiboot2 support to gummiboot for passing data to the trusted boot module
- does not impact the existing gummiboot functionality

Tboot
http://sourceforge.net/projects/tboot/