sd-image-raspberrypi4: mount boot partition

This should have been done initially, as otherwise it gets awfully
awkward to boot into new generations by default.

This system-specific image wasn't expected to be long-lived, thus why it
didn't end up being polished much.

Reality shows us we may be stuck with it for a bit longer, so let's make
it easier to use for new users.

nixos/modules/installer/cd-dvd/sd-image-raspberrypi4.nix

@@ -18,6 +18,7 @@
 
   sdImage = {
     firmwareSize = 128;
+    firmwarePartitionName = "NIXOS_BOOT";
     # This is a hack to avoid replicating config.txt from boot.loader.raspberryPi
     populateFirmwareCommands =
       "${config.system.build.installBootLoader} ${config.system.build.toplevel} -d ./firmware";
@@ -25,6 +26,12 @@
     populateRootCommands = "";
   };
 
+  fileSystems."/boot/firmware" = {
+    # This effectively "renames" the loaOf entry set in sd-image.nix
+    mountPoint = "/boot";
+    neededForBoot = true;
+  };
+
   # the installation media is also the installation target,
   # so we don't want to provide the installation configuration.nix.
   installer.cloneConfig = false;