nixos/filesystems: await builtin pstore module backend in mount-pstore
nixos/filesystems: await builtin pstore module backend in mount-pstore

If the pstore module is builtin, it nonetheless can take considerable
time to register a backend despite /sys/fs/pstore already appearing
mounted, so the condition is moved into the main script to extend
waiting for the backend to this case.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs