fix: resolve grub-pc / grub-efi-amd64 "held broken packages" conflict

The edition package lists installed BOTH grub-pc and grub-efi-amd64
(+ shim-signed). Those provide the same bootloader role and conflict in
apt, so every rootfs build failed with "unable to correct problems, you
have held broken packages".

A rootfs now carries exactly ONE bootloader, chosen by the BOOT variable
(mirroring the existing --boot bios|efi deploy option):

- versions.mk / common.sh: BOOT := bios (bios -> grub-pc,
  efi -> grub-efi-amd64 + shim-signed + mokutil), exported via the
  Makefile.
- build-rootfs.sh validates BOOT early and injects the matching boot
  packages into the apt install; the static package lists no longer
  contain any grub package.
- deploy-disk.sh / build-image.sh / install.sh default --boot from the
  same BOOT variable, so a rootfs and the artifact deployed from it can
  never disagree (BOOT=efi make image-cloud produces a UEFI image).
- mokutil is now installed explicitly in the efi flavour (it was not
  pulled in because we install with --no-install-recommends).
- docs updated (building.md knob + rationale, secureboot.md note).

docs/secureboot.md

@@ -53,9 +53,10 @@ against your MOK.
 
 ## Notes
 
-- Requires `sbsigntool` on the build host and `mokutil` in the image
-  (`mokutil` is pulled in by the `shim-signed` package already in the package
-  lists).
+- Requires `sbsigntool` on the build host. `mokutil` (for enrollment) is
+  installed in the image as part of the EFI boot flavour:
+  `BOOT=efi make iso-server` (the `efi` flavour adds `grub-efi-amd64
+  shim-signed mokutil`).
 - Losing `MOK.priv` means you cannot sign future updates — back it up.
 - Full vendor CA / Microsoft KEK signing is intentionally not used; revisit
   only if a commercial distribution is ever pursued.