initrd: boot dd-written hybrid ISOs in mount-usb and usb-autoboot

[tlaurion]

Summary

Fixes #2008: a Kicksecure hybrid ISO dd'd to a USB drive
now boots in both the manual USB boot menu and Basic-mode auto-boot.

The problem

A dd-written Kicksecure ISO is a GPT hybrid whose bootable iso9660
filesystem lives on the whole raw device, not on a partition. The boot
path relied on blkid TYPE="iso9660" to detect such drives, which does
not reliably report the type on these oversized/GPT-hybrid sticks, so no
bootable filesystem was found and boot failed.

The fix

• initrd/etc/functions.sh: add first_mountable_usb_disk() (a
  non-destructive read-only mount probe that finds a bootable filesystem
  on a whole device) and is_whole_disk(); host _get_blkid_fstype()
  (moved from kexec-iso-init.sh).
• initrd/bin/mount-usb.sh: add a --whole-disk flag that probes whole USB
  disks with an actual read-only mount instead of blkid; the default
  remains partitions-only.
• initrd/bin/media-scan.sh: the USB boot path opts in via
  mount_usb --whole-disk, and retries once with the default
  partitions-only mount when a whole disk mounts but yields no boot
  entries.
• initrd/etc/gui_functions.sh: mount_usb() forwards its arguments to
  mount-usb.sh and maps exit 5 (picker abort) to exit 1.
• initrd/bin/usb-autoboot.sh: enumerate whole disks before partitions so
  a dd-written hybrid ISO auto-boots in Basic mode.
• config/busybox.config: enable CONFIG_FEATURE_BLKID_TYPE=y so blkid
  reports the filesystem TYPE used by kexec-iso-init.sh.
• boards/qemu-coreboot-fbwhiptail-tpm2-basic-usb-autoboot: new QEMU test
  board (Basic mode + USB autoboot) for this path.
• tests/iso-test/: move the iso-boot test suite out of initrd/ so it no
  longer ships in the initrd image.
• Docs (boot-process.md, busybox_perks.md, iso_boot.md, logging.md).

Testing

• ISO boot test suite: 26 passed / 0 failed / 0 skipped.
• QEMU: a dd'd Kicksecure ISO mounts the whole device and reaches the
  boot menu in manual mode, and auto-boots in Basic mode; Tails, Debian
  live, and partitioned images (NixOS) still boot through the existing
  paths.

Boot confirmation and one-time runtime edit

• The dd'ed-USB boot path now reuses the ISO boot confirmation codepath: the full kexec command line is displayed and confirmed before boot (skip_confirm removed from the bootable-USB path in media-scan.sh).
• Added an Edit [e] option to that confirmation: edit the boot entry in vi as a one-time runtime change (nothing persisted to /boot or config.user), then re-show the final kexec command line for reconfirmation.
• make default (d) is suppressed after an edit, since a persisted edited entry would diverge from the on-media boot config and the signed /boot state.
• Added unit tests for edited-entry parse, validation, and the edited-append round-trip.

[Copilot]

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[tlaurion]

Verified working without regression under qemu on thumb drives with multiple isos dropped on it and directly booting on iso from qemu: both work.

[tlaurion]

verified dd'ed and iso boot on debian-13 xfce too.

[Copilot]

Pull request overview

Copilot reviewed 5 out of 7 changed files in this pull request and generated no new comments.

[tlaurion]

Was testing Kicksecure-LXQt-18.1.4.2.Intel_AMD64.iso, will test Kicksecure-LXQt-18.2.1.9.Intel_AMD64.iso

[tlaurion]

Tested with oversized raw image vs iso size (thumb drives should always be bigger then iso...) and now works as expected.

[Copilot]

Pull request overview

Copilot reviewed 11 out of 16 changed files in this pull request and generated no new comments.

Suppressed comments (2)

tests/iso-test/README.md:34

• This entry lists a Debian 13 DVD date of 2026-12-15, which is in the future relative to the current date. If this is meant to be the download/build date used for testing, it should reflect an actual past date (or be marked as TBD/unknown).
  initrd/etc/gui_functions.sh:31
• The abort/return handling here is broken because the failure branch runs in a subshell (...): USB_FAILED=1 won’t persist to the parent shell, and exit 1 exits only the subshell. This can leave USB_FAILED unset and make [ $USB_FAILED -ne 0 ] error, and it won’t reliably abort on picker exit code 5 as intended.

Capture the exit status in the parent shell and set USB_FAILED deterministically (and exit 1 from the parent) instead of using a subshell.

	# Forward "$@" (e.g. --whole-disk) to mount-usb.sh. It exits 5 when
	# the user aborts the USB disk picker; map that to exit 1 (abort).
	mount-usb.sh "$@" && USB_FAILED=0 || ([ $? -eq 5 ] && exit 1 || USB_FAILED=1)
	if [ $USB_FAILED -ne 0 ]; then
		whiptail_error --title 'USB Drive Missing' \
			--msgbox "Insert your USB drive and press Enter to continue." 0 80
		# mount-usb.sh exits 5 when the user aborts the USB disk picker; map that to exit 1 (abort).
		mount-usb.sh "$@" && USB_FAILED=0 || ([ $? -eq 5 ] && exit 1 || USB_FAILED=1)
		if [ $USB_FAILED -ne 0 ]; then