fix(convert): support GCP-style images with a separate ESP grub.cfg and vendor kernels

[Wilbert957]

Problem

cryptpilot-convert cannot produce a bootable image from a GCP Ubuntu 24.04 cloud image. Two independent issues:

1. Stale ESP grub.cfg. GCP images keep a full, firmware-read grub.cfg on the EFI System Partition (/boot/efi/EFI/<distro>/grub.cfg), while convert only regenerates the boot-partition copy (/boot/grub/grub.cfg). After conversion the ESP copy still references the pre-conversion kernel, so the firmware boots a stale (often already-removed) kernel and the boot fails with error: file '/vmlinuz-...' not found / error: file '/EFI/ubuntu/x86_64-efi/bli.mod' not found.

2. zram module detection hard-codes -generic. install_zram_module_if_needed greps dpkg -l for linux-image-...-generic. On an image whose only kernel is a cloud-vendor flavor (e.g. -gcp, -aws), the match is empty and convert aborts with Could not determine kernel version.

Fix

1. After regenerating the boot-partition grub.cfg, propagate it (and the grub module directory, e.g. x86_64-efi) to each separate ESP grub.cfg. No-op when there is no separate ESP copy; skips the case where grub-mkconfig already wrote straight to the ESP (e.g. the alinux symlink), detected with -ef.
2. Select the highest-versioned installed vmlinuz regardless of flavor, matching the kernel already chosen for the initrd later (ls /boot/vmlinuz-* | sort -V | tail -1).

Both changes are scoped to behavior that was already broken for these images and are no-ops on layouts that already worked (e.g. alinux UEFI, single-grub.cfg images).

Testing

End-to-end on a GCP Ubuntu 24.04 image converted with a 6.17.0-1018-gcp kernel, with the equivalent out-of-tree post-processing removed so convert alone is exercised:

• convert logs Syncing regenerated grub.cfg to ESP: /boot/efi/EFI/ubuntu/grub.cfg; the ESP grub.cfg default entry now points at the -gcp kernel and the x86_64-efi module dir (incl. bli.mod) is present.
• zram detection resolves 6.17.0-1018-gcp instead of aborting.
• The converted image boots successfully both under QEMU/OVMF and on a real GCP TDX Confidential VM: cryptpilot-fde before-sysroot sets up dm-verity + zram + dm-snapshot, /sysroot mounts, after-sysroot completes, switch-root succeeds, and the system reaches multi-user.

[shankailun-aliyun]

@Wilbert957 ，您好，您的请求已接收，请耐心等待结果。

[shankailun-aliyun]

@Wilbert957 ，您好，未检测到有镜像需要构建，如需重新检测请评论 /start 。