Failures in `mmd` and `mcopy` are silently ignored during build

[klange]

When building for x86-64, a FAT image is created and then populated with mmd and mcopy to serve as the EFI El Torito payload.

This initial FAT image has a fixed size, haphazardly calculated from host disk usage of the source directory. This calculation is prone to underestimating the necessary size, which generally results in the attempt to add the ramdisk failing. As this is currently ignored, the build will conclude with a false success, but the resulting CD image will only have a kernel.

It would be better if this was caught so that CI builds can fail in this case.