Add UEFI target (*.efi)

[oech3]

I don't know how to do that.

[Ecordonnier]

Can you please add some details? What do you mean with uefi target?

[oech3]

Allow to run cksum.efi on UEFI shell (shellx64.efi).

[oech3]

clap and no_std are large barriers for UEFI target.