Added support for target-specific config metadata

[trashbyte]

Just a quick little change to allow bootimage to work with aarch64. I haven't been able to actually test it because I don't have a working aarch64-compatible bootloader (im using bootloader which is x86 only). Really I was just experimenting to see how much work it would be to add a raspi target to my toy OS, which I don't think I'm going to follow through with. I do know default x86_64 behavior still works as expected. Merge the changes if you like, reject them if you don't, I don't mind either way. Just figured I'd share it in case it's useful.