bin/seed_package_mirror.sh: dynamically seed all boards & toolchains

[tlaurion]

Fixes #2177

Problem

bin/seed_package_mirror.sh seeded the Purism package mirror using a
hardcoded list of five representative boards:

• qemu-coreboot-fbwhiptail-tpm1-hotp (base x86)
• UNTESTED_talos-2 (PPC)
• librem_l1um_v2 (TPM2)
• EOL_librem_l1um (coreboot 4.11)
• EOL_x230-maximized (io386)

After #2174 moved the musl-cross-make component tarballs under
packages/, the mirror did not get all of them cached on demand,
so builds served those 7 tarballs from the Purism mirror (MIRROR_USED
in build/mirror_fallbacks.log) rather than from primary sources.

The hardcoded list is also fragile: boards are added/renamed and
coreboot forks change over time, so the mirror was always at risk of
silently missing newly added module tarballs.

Change

Rewrite bin/seed_package_mirror.sh to:

• Discover all boards dynamically via boards/*/*.config instead of a
  hardcoded list, so new boards, architectures, and coreboot forks are
  picked up automatically with no maintenance.
• Run make BOARD=<board> packages for every board. The define_module
packages: target pre-downloads every versioned module tarball plus the
  coreboot crossgcc toolchain tarballs, which is what actually gets copied
  into the mirror.
• Clean only the cached tarballs (packages/*/*) and .canary build
  stamps (build/**/.canary, modeled on the
  real.remove_canary_files-extract_patch_rebuild_what_changed Makefile
  helper) so module tarballs are re-downloaded + hash-verified and
  coreboot forks re-emit their crossgcc toolchain, without wiping already
  compiled artifacts.
• Redirect make stdin from /dev/null to avoid an intermittent
  bash: syntax error near unexpected token 'do' that occurred under
  set -e + if ! make ... when make/wget consumed the script's stdin.
• Tolerate per-board failures (e.g. a dead upstream URL such as the
  coreboot 4.11 acpica tarball) without aborting the whole seed,
  reporting a summary of failed boards at the end.
• Copy tarballs arch-agnostically (packages/*/) instead of
  hardcoding x86/ppc64.

Verification

• A full run over all boards completes with exit 0.
• Mirror contains the complete tarball set (90 files), including all 7
  musl-cross-make component tarballs that were the core of Make sure Purism cache all tarballs from merged https://github.com/linuxboot/heads/pull/2174 #2177.
• build/mirror_fallbacks.log shows the 7 musl-cross-make tarballs are now
  fetched from primary sources (PRIMARY) instead of MIRROR_USED.
• Coreboot forks are not fully re-cloned: clearing their .canary
  re-runs the Makefile branch that re-fetches the pinned commit and
  regenerates the .canary (observed: .canary present after the run,
  fork dirs preserved), yielding the crossgcc toolchain tarballs without a
  full re-clone.

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[tlaurion]

@JonathonHall-Purism should be future proof and make sure everything currently maintained is mirrored.