Make hermes build fetching more flavour resilient (facebook#41055)

Summary:
Pull Request resolved: facebook#41055

This should fix the issue highlighted in [this comment](reactwg/react-native-releases#64 (comment)).
Basically, before the fix, we were not supporting flavours correctly, as we assumed that only Debug and Release were available.

With this change, we infer whether we have to fetch Hermes for Debug or Release based on the actual flags that are passed. In this way, the users can customize their app's flavors more freely.

## Changelog:
[Internal] - Support multiple flavors when downloading Hermes

Reviewed By: huntie

Differential Revision: D50408381

fbshipit-source-id: 6990218b286b4dd823323bc63de90279efc9e74e

Author: cipolleschi

packages/react-native/sdks/hermes-engine/hermes-engine.podspec

@@ -60,7 +60,13 @@ Pod::Spec.new do |spec|
         :execution_position => :before_compile,
         :script => <<-EOS
         . "$REACT_NATIVE_PATH/scripts/xcode/with-environment.sh"
-        "$NODE_BINARY" "$REACT_NATIVE_PATH/sdks/hermes-engine/utils/replace_hermes_version.js" -c "$CONFIGURATION" -r "#{version}" -p "$PODS_ROOT"
+
+        CONFIG="Release"
+        if echo $GCC_PREPROCESSOR_DEFINITIONS | grep -q "DEBUG=1"; then
+          CONFIG="Debug"
+        fi
+
+        "$NODE_BINARY" "$REACT_NATIVE_PATH/sdks/hermes-engine/utils/replace_hermes_version.js" -c "$CONFIG" -r "#{version}" -p "$PODS_ROOT"
         EOS
       }
     end