Use GRUB for USB boot on EFI 64-bit

We need this for Secure Boot (#6560) and it’ll simplify quite a few parts of our code.

Making a Tails USB image boot with GRUB is straightforward but we need to:

• Find another way to identify the boot device in our partitioning script: git grep -E 'FSUUID|sysappend'
• Ensure this does not break stuff on legacy BIOS boot, 32-bit EFI, nor DVD boot
• Update our test suite accordingly: it currently expects the syslinux UI
• Update our documentation about editing the kernel command line accordingly
• Update our design doc accordingly

Team: FT

Feature Branch: feature/6560-secure-boot

Related issues

• Related to #7422 (closed)
• Related to #15292 (closed)
• Related to #12440 (closed)
• Related to #12439 (closed)
• Related to #16229
• Blocks #6560
• Blocks #16209
• Blocks #16980
• Blocks #17049

Original created by @intrigeri on 15806 (Redmine)