nixos: Generalise the container tests in stage-2 boot

This way, stage-2 behaves correctly also for libvirt-lxc containers.

Some more discussion on this:
https://github.com/NixOS/nixpkgs/commit/a7a08188bf650ababa36300a9a6f34169e2a73bf
https://github.com/NixOS/nixpkgs/commit/bfe46a653ba2f8ff9902128f485cbd87c49cbca7

nixos/modules/system/boot/stage-2-init.sh

@@ -29,14 +29,14 @@ setPath "@path@"
 # Normally, stage 1 mounts the root filesystem read/writable.
 # However, in some environments, stage 2 is executed directly, and the
 # root is read-only.  So make it writable here.
-if [ "$container" != systemd-nspawn ]; then
+if [ -z "$container" ]; then
     mount -n -o remount,rw none /
 fi
 
 
 # Likewise, stage 1 mounts /proc, /dev and /sys, so if we don't have a
 # stage 1, we need to do that here.
-if [ ! -e /proc/1 ]; then
+if [ ! -e /proc/1 ] || [ -n "$container" ]; then
     specialMount() {
         local device="$1"
         local mountPoint="$2"