iso: prefer dracut, append splash quiet, take initial snapper snapshot

[dongioia]

Wires up dracut, Plymouth, and an initial snapper snapshot in the from_iso install flow.

Depends on beryllium-org/Beryllium-ISO#1. The dracut path only fires when the unpacked airootfs has /usr/bin/dracut, so this can land first; ISOs without it fall through to the existing mkinitcpio -P call.

Changes

bakery/iso.py:

• regenerate_initramfs: if /usr/bin/dracut exists in the target, run dracut --regenerate-all --force and delete the stale initramfs-linux.img left over from the live ISO so GRUB doesn't pick the mkinitcpio image. Otherwise the existing mkinitcpio -P path runs.
• grub_cfg: new cmdline_append parameter. Reads the current GRUB_CMDLINE_LINUX_DEFAULT, appends the new tokens, dedups so a second run doesn't double up. Used to add splash quiet without clobbering board-specific cmdlines like console=ttyS2,1500000n8 on Rock 5B+.
• take_initial_snapshot: best-effort. Bails out when snapper isn't installed. Wraps snapper -c root create-config / and the baseline snapper create -d "Initial install" in || true so non-btrfs installs don't error.

bakery/install.py:

• Calls grub_cfg(cmdline_append="splash quiet", update=False, ...) before grub_install. update=False skips a redundant grub-mkconfig pass since grub_install runs one with the new cmdline anyway.
• Calls take_initial_snapshot(mnt_dir) after final_setup and before unmount_all, while /.snapshots is still mounted.

Backward compat

• ISOs without dracut: fall back to mkinitcpio. No change for them.
• Existing grub_cfg(cmdline=...) callers: unchanged. cmdline still wins over cmdline_append when both are passed.
• take_initial_snapshot: returns early without snapper. Non-btrfs installs ignored.

Test plan

• mainline-iso aarch64 once Beryllium-ISO#1 rebuilds: from_iso install on Rock 5B+ btrfs subvol
• After install: /boot/initramfs-${KVER}.img present, no leftover initramfs-linux.img
• grep CMDLINE /etc/default/grub shows splash quiet appended after the existing tokens
• snapper -c root list lists the baseline snapshot
• First boot: Plymouth splash shows during init, greeter loads
• Older mkinitcpio-only ISO: install still works the same

[dongioia]

Thanks. Pushed a follow-up commit (554eee1) that calls dracut-rebuild when present and falls back to dracut --regenerate-all --force otherwise. Two commits on the branch, happy to squash before merge if you prefer.

No rush, ping when you get to it.