hadrian: Flag cabal flag handling

This patch basically deletes some ad-hoc handling of Cabal Flags and
replaces it with a correct query of the LocalBuildInfo. The flags in the
local build info can be modified by users by passing hadrian options

For example (!4331)

```
*.genapply.cabal.configure.opts += --flags=unregisterised
```

And all the flags specified by the `Cabal Flags` builder were already
passed to configure properly using `--flags`.

(cherry picked from commit c832ae93)

hadrian/src/Hadrian/Haskell/Cabal/Parse.hs

@@ -213,20 +213,15 @@ resolveContextData context@Context {..} = do
     -- Configure the package with the GHC for this stage
     (compiler, platform) <- configurePackageGHC package stage
 
-    flagList <- interpret (target context (Cabal Flags stage) [] []) =<< args <$> flavour
-    let flags = foldr addFlag mempty flagList
-          where
-            addFlag :: String -> C.FlagAssignment -> C.FlagAssignment
-            addFlag ('-':name) = C.insertFlagAssignment (C.mkFlagName name) False
-            addFlag ('+':name) = C.insertFlagAssignment (C.mkFlagName name) True
-            addFlag name       = C.insertFlagAssignment (C.mkFlagName name) True
-
-    let (Right (pd,_)) = C.finalizePD flags C.defaultComponentRequestedSpec
-                         (const True) platform (C.compilerInfo compiler) [] gpd
 
     cPath <- Context.contextPath context
     lbi <- liftIO $ C.getPersistBuildConfig cPath
 
+    let flags = C.flagAssignment lbi
+
+        (Right (pd,_)) = C.finalizePD flags C.defaultComponentRequestedSpec
+                         (const True) platform (C.compilerInfo compiler) [] gpd
+
     -- Note: the @cPath@ is ignored. The path that's used is the 'buildDir' path
     -- from the local build info @lbi@.
     pdi <- liftIO $ getHookedBuildInfo [pkgPath package, cPath -/- "build"]