extract phram.mtd ramdisk support from tftpboot module

also rename tftproot output to tftpboot for consistency

modules/ramdisk.nix

@@ -0,0 +1,34 @@
+{
+  config
+, pkgs
+, lib
+, ...
+}:
+let
+  inherit (lib) mkIf mkEnableOption mkOption; # types concatStringsSep;
+in {
+  options = {
+    boot = {
+      ramdisk = {
+        enable = mkEnableOption (lib.mdDoc ''
+          Configure kernel to enable reserving part of memory as
+          an MTD-based RAM disk.  Needed for TFTP booting or for
+          kexec-based revertable upgrade
+        '');
+      };
+    };
+  };
+  config = mkIf config.boot.ramdisk.enable {
+    kernel = {
+      config = {
+        MTD = "y";
+        MTD_PHRAM = "y";
+        MTD_CMDLINE_PARTS = "y";
+        MTD_OF_PARTS = "y";
+        PARTITION_ADVANCED = "y";
+        MTD_BLKDEVS = "y";
+        MTD_BLOCK = "y";
+      };
+    };
+  };
+}