refactor: Change boot type JSON representation to allow easier implementations in other languages

[lj3954]

BootType can currently produce this JSON: "boot": "legacy" or "boot": "efi" { secure_boot: true/false }
This is difficult to represent in other languages, and should be modified. This issue concerns quickget_cigo.

A potential solution would be to change it to a simple enum, with the possible values being 'legacy', 'efi', 'secure_efi' or something of the like.