mkspiffs: fix version number shown in help, cleanup

* Replace git invocation for the project itself with the version number from the derivation
* Replace git invocation for the submodule with 'unknown' (which is what the Makefile falls back to anyway)
* Remove git as a dependency
* Update sha256 -> hash

pkgs/tools/filesystems/mkspiffs/default.nix

···
-{ lib, stdenv, fetchFromGitHub, git }:
+{ lib, stdenv, fetchFromGitHub }:
 
 # Changing the variables CPPFLAGS and BUILD_CONFIG_NAME can be done by
 # overriding the same-named attributes. See ./presets.nix for examples.
···
     repo = "mkspiffs";
     rev = version;
     fetchSubmodules = true;
-    sha256 = "1fgw1jqdlp83gv56mgnxpakky0q6i6f922niis4awvxjind8pbm1";
+    hash = "sha256-oa6Lmo2yb66IjtEKkZyJBgM/p7rdvmrKfgNd2rAM/Lk=";
   };
 
-  nativeBuildInputs = [ git ];
   buildFlags = [ "dist" ];
+
+  makeFlags = [
+    "VERSION=${version}"
+    "SPIFFS_VERSION=unknown"
+  ];
+
   installPhase = ''
     mkdir -p $out/bin
     cp mkspiffs $out/bin