Is your feature request related to a problem? Please describe.
When using a setup like the one suggested by @hnez in #1696 (comment), some UEFI
implementations tend to remove boot entries if they point to unmountable
filesystems (which can happen due to a power cycle while installing).

Describe the solution you'd like
We should add the missing information to the slot sections in the system.conf so that RAUC can recreate entries if they are missing.

[slot.esp.0]
device=/dev/sda1
bootname=system0
efi-loader=\\EFI\\BOOT\\BOOTX64.EFI
efi-unicode="@1" # UKI profile 1

[slot.esp.1]
device=/dev/sda2
bootname=system1
efi-loader=\\EFI\\BOOT\\BOOTX64.EFI
efi-unicode="@2" # UKI profile 2

Describe alternatives you've considered
The current workaround is to use efibootmgr from a post-install hook.
As probably many UEFI implementations are affected, this should be moved to common code in RAUC.