bug fixes, and support gummiboot

Changelog for this series:

• fixes / prevents a class of bugs related to having upper case characters in deviceinfo vars when they are used for string comparison.

• fixes some issues where boot-deploy would fail and leave half-installed state in the output directory, by changing some functions to append to additional_files rather than having them call copy directly.

• removes hardcoding of paths for some functions that were copying to the output dir

• adds support for gummiboot (and would be trivial to extend to support systemd-boot later). Why? Because this bootloader is a lot better than using grub to support EFI booting in some distros (pmOS...) and it gets us a little bit closer to maaayyybe supporting secure boot on some platforms in the distant future. This adds a new deviceinfo variable, deviceinfo_generate_gummiboot