Separate backup

Recommended to review by commit as I try to structure it as refactoring goes. My intention is also to use it as tutoring material for some refactoring presentation.
yeah, I also do not have strong opinion what is nicer name. naming it Backup is to ambition. This class handle only br ( boot record) backup. We also have handle file configuration backups, but I want to have it separate as it is different purpose. Maybe `Bootloader::BackupBR.create_for(device)`. What do you think?