Let any bootstrap key suffice to allow features

This is a follow-up to pull request rust-lang#32812, according to the new
decision in issue rust-lang#36548 to relax bootstrap key logic.  Now *any*
non-empty bootstrap key is allowed to circumvent the feature gate.

For now, the build system still goes through the same motions to set
RUSTC_BOOTSTRAP_KEY according to the prior system.  This is necessary
at least until the next stage0 base includes this relaxed key check.
It also makes it simpler to revert this if we change our mind.

src/libsyntax/feature_gate.rs

@@ -1317,15 +1317,13 @@ impl UnstableFeatures {
     pub fn from_environment() -> UnstableFeatures {
         // Whether this is a feature-staged build, i.e. on the beta or stable channel
         let disable_unstable_features = option_env!("CFG_DISABLE_UNSTABLE_FEATURES").is_some();
-        // The secret key needed to get through the rustc build itself by
-        // subverting the unstable features lints
-        let bootstrap_secret_key = option_env!("CFG_BOOTSTRAP_KEY");
-        // The matching key to the above, only known by the build system
-        let bootstrap_provided_key = env::var("RUSTC_BOOTSTRAP_KEY").ok();
-        match (disable_unstable_features, bootstrap_secret_key, bootstrap_provided_key) {
-            (_, Some(ref s), Some(ref p)) if s == p => UnstableFeatures::Cheat,
-            (true, _, _) => UnstableFeatures::Disallow,
-            (false, _, _) => UnstableFeatures::Allow
+        // Check whether we want to circumvent the unstable feature kill-switch for bootstrap
+        let bootstrap_circumvent = env::var_os("RUSTC_BOOTSTRAP_KEY")
+            .map_or(false, |e| !e.is_empty());
+        match (disable_unstable_features, bootstrap_circumvent) {
+            (_, true) => UnstableFeatures::Cheat,
+            (true, _) => UnstableFeatures::Disallow,
+            (false, _) => UnstableFeatures::Allow
         }
     }