rfs(firmware): pack full Alpine linux-firmware set from container and overmount /lib/firmware

Change pack source: install all linux-firmware* packages in container and pack from /lib/firmware via [bash.rfs_common_install_all_alpine_firmware_packages()](scripts/rfs/common.sh:290) used by [bash.pack-firmware.sh](scripts/rfs/pack-firmware.sh:21). At runtime, overmount firmware flist on /lib/firmware by updating [sh.firmware.sh](config/zinit/init/firmware.sh:10). Update docs to reflect /lib/firmware mount and new pack strategy.

scripts/rfs/common.sh

@@ -287,6 +287,92 @@ rfs_common_validate_modules_metadata() {
   fi
 }
 
+# -----------------------------------------------------------------------------
+# Alpine firmware installation in container (for flist packing)
+# -----------------------------------------------------------------------------
+# Install all available linux-firmware* packages into the build container root.
+# This prepares a full /lib/firmware tree to be packed into an RFS flist.
+# Notes:
+# - Requires root inside the container (the builder image runs scripts as root).
+# - Uses apk search to enumerate all linux-firmware packages; installs them all.
+rfs_common_install_all_alpine_firmware_packages() {
+  log_info "Installing all Alpine linux-firmware* packages into container (/lib/firmware)"
+  # Ensure apk index is fresh
+  safe_execute apk update
+
+  # Enumerate firmware packages. Output of 'apk search' looks like:
+  #   linux-firmware-rtl_nic-20231030-r0
+  # Strip version suffix to get the package names acceptable to apk add.
+  local pkgs_raw
+  pkgs_raw="$(apk search 'linux-firmware*' 2>/dev/null || true)"
+
+  if [[ -z "${pkgs_raw// }" ]]; then
+    log_warn "No linux-firmware* packages found via apk search"
+  fi
+
+  # Build unique package list without versions
+  # 1) take first column (package-with-version)
+  # 2) strip trailing '-<digits...>' (version/revision)
+  # 3) dedupe
+  local pkgs=()
+  while IFS= read -r line; do
+    [[ -z "${line// }" ]] && continue
+    local name="${line%% *}"
+    name="$(echo "$name" | sed -E 's/-[0-9].*$//')"
+    if [[ -n "$name" ]]; then
+      pkgs+=("$name")
+    fi
+  done <<< "$pkgs_raw"
+
+  if [[ ${#pkgs[@]} -eq 0 ]]; then
+    log_warn "Firmware package list is empty after parsing; attempting base meta-package 'linux-firmware'"
+    pkgs=("linux-firmware")
+  fi
+
+  # Deduplicate while preserving order
+  local seen="" final_pkgs=()
+  for p in "${pkgs[@]}"; do
+    if ! grep -qx "$p" <<< "$seen"; then
+      final_pkgs+=("$p")
+      seen+=$'\n'"$p"
+    fi
+  done
+
+  log_info "Installing ${#final_pkgs[@]} firmware packages:"
+  for p in "${final_pkgs[@]}"; do
+    log_info "  - $p"
+  done
+
+  # Install all firmware packages; allow some failures (not all subpackages exist on all arches)
+  local failed=()
+  for p in "${final_pkgs[@]}"; do
+    if apk add --no-cache "$p" >/dev/null 2>&1; then
+      log_debug "Installed: $p"
+    else
+      log_warn "Failed to install: $p"
+      failed+=("$p")
+    fi
+  done
+
+  # Quick check that /lib/firmware exists and is populated
+  if [[ -d "/lib/firmware" ]]; then
+    local cnt
+    cnt=$(find /lib/firmware -type f | wc -l || echo 0)
+    log_info "/lib/firmware population: ${cnt} files"
+    if [[ "$cnt" -eq 0 ]]; then
+      log_warn "/lib/firmware exists but is empty after installation"
+    fi
+  else
+    log_error "/lib/firmware directory not found after firmware installation"
+    return 1
+  fi
+
+  if [[ ${#failed[@]} -gt 0 ]]; then
+    log_warn "Some firmware packages failed to install (${#failed[@]} failures); proceeding with available set"
+  fi
+  return 0
+}
+
 # -----------------------------------------------------------------------------
 # Manifest patching (sqlite .fl)
 # -----------------------------------------------------------------------------