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).

Makefile

@@ -28,7 +28,7 @@ include versions.mk
 # Pass edition + env through to the scripts.
 export DISTRO_NAME DISTRO_ID VERSION RELEASE_NAME
 export DEBIAN_SUITE DEBIAN_MIRROR SECURITY_MIRROR ARCH
-export KERNEL_PACKAGE KERNEL_VERSION
+export KERNEL_PACKAGE KERNEL_VERSION BOOT
 export BUILD_DIR ROOTFS_DIR IMAGE_DIR DEB_DIR LOG_DIR ARTIFACT_DIR
 export TOOLCHAIN_REPO
 
@@ -55,7 +55,7 @@ help:
 	@echo
 	@echo "Configuration (see versions.mk):"
 	@echo "  VERSION=$(VERSION)  DEBIAN_SUITE=$(DEBIAN_SUITE)  ARCH=$(ARCH)"
-	@echo "  ARCLINE_TOOLCHAIN=auto|skip|require  (toolchain in image builds)"
+	@echo "  BOOT=$(BOOT)  ARCLINE_TOOLCHAIN=auto|skip|require  (bootloader / toolchain in image builds)"
 
 # ── host deps ────────────────────────────────────────────────────────────────
 deps: