Fix oss buck build

Summary:
We need to fix the following things in order to build:

1. properly link prelude to `.buckconfig`
2. add a shim version of `prelude/BUCK` in `shim/BUCK`, so that dependencies like `toolchains//:cxx` can find its way.
3. bring `env_interface.bzl` up-to-date. The internal version of it has more attributes and we need to keep them in sync.
4. patch .gitignore so it doesn't ignore any path containing `build/`, for example, `shim/xplat/executorch/build/`

Reviewed By: cccclai, dbort

Differential Revision: D47642414

fbshipit-source-id: a3803339b6fe7859035bdf4ceb41469aa0f7f0c7

Author: larryliu0820

.buckconfig

@@ -6,7 +6,7 @@
 
 [repositories]
   root = .
-  prelude = prelude
+  prelude = third-party/prelude
   shim = shim
 
 [repository_aliases]

.gitignore

@@ -1,3 +1,3 @@
-build/
+buck-out/
 executorch.egg-info
 __pycache__/

shim/xplat/executorch/build/env_interface.bzl

@@ -10,6 +10,10 @@ load(":type_defs.bzl", "is_list", "is_tuple")
 
 _ET_TARGET_PREFIX = "executorch"
 
+# Indicates that an external_dep entry should fall through to the underlying
+# buck rule.
+_EXTERNAL_DEP_FALLTHROUGH = "<fallthrough>"
+
 _EXTERNAL_DEPS = {
     # ATen C++ library deps
     "aten-core": [],  # TODO(larryliu0820): Add support
@@ -185,4 +189,5 @@ env = struct(
     remove_platform_specific_args = _remove_platform_specific_args,
     resolve_external_dep = _resolve_external_dep,
     target_needs_patch = _target_needs_patch,
+    EXTERNAL_DEP_FALLTHROUGH = _EXTERNAL_DEP_FALLTHROUGH,
 )