Warn about the use of deprecated env() function

This patch enables warnings if one of the deprecate functions that rely
in the implicit use of the global Genode::env() accessor are called.

For the time being, some places within the base framework continue
to rely on the global function while omitting the warning by calling
'env_deprecated' instead of 'env'.

Issue #1987

repos/base-fiasco/src/lib/base/thread_bootstrap.cc

@@ -37,5 +37,5 @@ void Thread::_thread_bootstrap() { }
 void Thread::_init_platform_thread(size_t, Type type)
 {
 	if (type == NORMAL) { return; }
-	_thread_cap = Genode::env()->parent()->main_thread_cap();
+	_thread_cap = Genode::env_deprecated()->parent()->main_thread_cap();
 }