feat(install): enable systemd-boot for ostree backend

[andrewdunndev]

Replace the bail!("bootupd is required for ostree-based installs") with working systemd-boot support for the ostree backend. The approach keeps ostree's ext4 /boot for its internal management, then copies BLS entries and kernel artifacts to the FAT32 ESP where systemd-boot can read them.

Changes

baseline.rs: Force a /boot partition when Bootloader::Systemd + non-composefs. Format it as ext4 (ostree needs POSIX fs for hardlinks/symlinks).

install.rs: Set sysroot.bootprefix=false for systemd-boot so BLS paths are relative to /boot root, not prefixed with /boot/. Replace the bail with install_systemd_boot() + sync_boot_to_esp().

bootloader.rs: New sync_boot_to_esp() function (~100 lines) that mounts the ESP, reads BLS entries from /boot/loader/entries/, and copies .conf files + referenced kernel + initrd to the ESP.

Partition layout

p1: 1M    BIOS boot
p2: 512M  ESP (FAT32) -- systemd-boot EFI binary + BLS entries + kernel/initrd
p3: 510M  /boot (ext4) -- ostree internal (hardlinks, symlinks, atomic swaps)
p4: rest  root (XFS)

Test results

GCP n2-standard-8, nested KVM, Fedora 42, QEMU + OVMF 4M:

Test	Result
Partition layout (BIOS + ESP + ext4 /boot + XFS root)	PASS
BLS entries with bootprefix=false	PASS
BLS + kernel + initrd copied to ESP	PASS
systemd-boot v257.11 boots system	PASS
ostree deployment active	PASS
bootc status correct (store=ostreeContainer)	PASS
kargs.d entries in /proc/cmdline	PASS
systemd-networkd working	PASS

Scope

This covers install-time support only. See #2098 for discussion of the upgrade-time ESP sync -- ostree staging does not write BLS entries to /boot until the next boot (ostree-finalize-staged.service), so the upgrade path needs a different approach.

Relates to #2098, #92

[gemini-code-assist]

Code Review

This pull request enables systemd-boot support for ostree-based installations by implementing a synchronization mechanism that copies boot artifacts from the ext4 /boot partition to the FAT32 ESP. Key changes include the addition of sync_boot_to_esp to manage BLS entries and kernel files, ensuring /boot is formatted as ext4 for POSIX compatibility, and configuring ostree with sysroot.bootprefix=false. Review feedback recommends enhancing the robustness of the BLS configuration parser to handle complex values and simplifying path expressions in the installation logic to reduce unnecessary cloning.