base: remove env_deprecated from thread-start code

This patch replaces the internal use 'env_deprecated()' from the
implementation of the thread API in the base library. It also
replaces the global accessor 'main_thread_cap' by the explicit
propagation of the main-thread's capability to the single point of
use via a new 'init_thread_bootstap' function.

Issue #4784

repos/base-linux/src/lib/lx_hybrid/lx_hybrid.cc

@@ -402,6 +402,11 @@ static void *thread_start(void *arg)
 }
 
 
+void Genode::init_thread(Cpu_session &, Region_map &)  { }
+void Genode::init_thread_start(Capability<Pd_session>) { }
+void Genode::init_thread_bootstrap(Thread_capability)  { }
+
+
 extern "C" void *malloc(::size_t size);
 extern "C" void free(void *);