release/amd64: cleanup code duplication

[VexedUXR]

Improve the release script by checking MK_LOADER_IA32 instead of the janky if -f "${BASEBITSDIR}/boot/loader_ia32.efi"

I'm hesitant about putting this in install-boot.sh, it feels out of place. Maybe a new script in release/amd64 that gets sourced by both make-memstick and mkisoimages would be better? I'm not sure..

This also makes the script complain if MK_LOADER_IA32 is yes but the file is missing, instead of silently not copying it. FWIW it also allows the user to build the ia32 loader, but omit it from the img, though I'm not sure what the use case would be.

I've been meaning to do this since 599273f landed but forgot, so here it is.

[VexedUXR]

Ok, went back to simply checking if the loader files exist. :)

[bsdimp]

@VexedUXR Can you commit or close? I think we should do this before 15.

[VexedUXR]

Ah, I forgot about this. I'll commit it.