workaround esp-idf version detection

If version.txt is missing some part of the cmake toolchain tries to read
refs/heads/fetchgit in the idf repo which failed.

This is not a perfect solution, but works nicely when using cargo.

pkgs/esp-idf/default.nix

@@ -120,6 +120,12 @@ stdenv.mkDerivation rec {
   dontUseNinjaCheck = true;
 
   installPhase = ''
+    # make esp-idf cmake git version detection happy
+    git init .
+    git config user.email "nixbld@localhost"
+    git config user.name "nixbld"
+    git commit --date="1970-01-01 00:00:00" --allow-empty -m "make idf happy"
+
     mkdir -p $out
     cp -rv . $out/