core: remove Core_env

This patch adjusts the last remaining callers of 'core_env' and removes
the 'Core_env' interface.

- Core's RAM/cap accounts are now represented by 'Core_account'
  implementing the 'Pd_account' interface.

- The former parts of 'Core_env' are now initialized in sequence
  in 'bootstrap_component'.

- 'Core_child' has been moved to a header to reduce the code in
  'main.cc' to a bare minimum. This as a preparation for the
  plan of making 'main.cc' specific for each kernel.

Fixes #5408

repos/base-linux/src/core/platform.cc

@@ -20,9 +20,9 @@
 #include <base/internal/capability_space_tpl.h>
 
 /* local includes */
-#include "platform.h"
-#include "core_env.h"
-#include "resource_path.h"
+#include <platform.h>
+#include <core_region_map.h>
+#include <resource_path.h>
 
 /* Linux includes */
 #include <core_linux_syscalls.h>