Fix repeated bootstrap runs

Updating the bootstrap script in #1689 made it so symlinks that already
exist return `exist` rather than ok, in line with the regular code.
However the bootstrap module wouldn't handle this case.

This fixes the problem by ensuring that `exists` is as valid as `ok`.

bootstrap

@@ -168,8 +168,9 @@ compile_file(File, Opts) ->
 bootstrap_rebar3() ->
     filelib:ensure_dir("_build/default/lib/rebar/ebin/dummy.beam"),
     code:add_path("_build/default/lib/rebar/ebin/"),
-    ok = symlink_or_copy(filename:absname("src"),
-                         filename:absname("_build/default/lib/rebar/src")),
+    Res = symlink_or_copy(filename:absname("src"),
+                          filename:absname("_build/default/lib/rebar/src")),
+    true = Res == ok orelse Res == exists,
     Sources = ["src/rebar_resource.erl" | filelib:wildcard("src/*.erl")],
     [compile_file(X, [{outdir, "_build/default/lib/rebar/ebin/"}
                      ,return | additional_defines()]) || X <- Sources],
@@ -648,4 +649,4 @@ join([H|T], Sep) ->
 chr(S, C) when is_integer(C) -> chr(S, C, 1).
 chr([C|_Cs], C, I) -> I;
 chr([_|Cs], C, I) -> chr(Cs, C, I+1);
-chr([], _C, _I) -> 0.
+chr([], _C, _I) -> 0.