Load shim_certificates.efi during netboot

[rosslagerwall]

Network boot cases do not support reading through a directory but it can still be useful to load unbundled certificates. Try to load them directly.

[jsetje]

The naming convention for those files is supposed to be "shim_certificate*.efi", but obviously that won't work without some sort of a readdir. Do you want to support a single, different filename for netboot? (Edit: this may well make sense, I just want to make sure it's intentional.)

Also, that will generate another line of noise when it's not there. I should probably just break up the interface to properly suppress that message. (I didn't want to risk that at the time when I created the initial netboot revocations.efi changes.)