Boot device detection is not deterministic

Please identify some basic details to help process the report

A. Provide Hardware Details

1. What board are you using? (Choose from the list of boards here)
   novacustom-v560tu

B. Identify how the board was flashed

3. How was Heads initially flashed?
   • External flashing
   • Internal-only / 1vyprep+1vyrain / skulls
   • Don't know

C. Identify the rom related to this bug report

1. Did you download or build the rom at issue in this bug report?

   • [x ] I downloaded it
   • I built it

2. If you downloaded your rom, where did you get it from?

   • Heads CircleCi
   • Purism
   • Nitrokey
   • Dasharo DTS (Novacustom)
   • Somewhere else (please identify)

   Please provide the release number or otherwise identify the rom downloaded

3. If you built your rom, which repository:branch did you use?

   • Heads:Master
   • Other (please identify)

Please describe the problem

Describe the bug
If in you have 2 nvme installed in the v56 laptop: same size and vendor, and both nvme are valid boot devices, heads will be switching between these boot devices every reboot in a random way. This creates a lot of different faulty behaviours which are hard to diagnose.

Expected behavior
Always choose the same boot device, warn at OEM-Factory-Reset that there are 2. valid Boot Devices and ask which one is the correct one.

Additional context
my guess is that this fdisk call https://github.com/linuxboot/heads/blob/d4c4e5699b89365a88d9d49748dbcc11b6394907/initrd/etc/functions#L1142 is to blame not sure if this is also a problem for non nvme setups

Start with the fdisk call in initrd/etc/functions around line 1142 and reproduce the report with two same-size, same-vendor NVMe devices. Trace how valid boot devices are selected across reboots, then verify that selection is deterministic and that OEM-Factory-Reset warns when two valid devices are present.