py: Move stack_ctrl_init() to mp_init().

As stack checking is enabled by default, ports which don't call
stack_ctrl_init() are broken now (report RuntimeError on startup). Save
them trouble and just init stack control framework in interpreter init.
diff --git a/py/runtime.c b/py/runtime.c
index b539984..f08ff9f 100644
--- a/py/runtime.c
+++ b/py/runtime.c
@@ -45,6 +45,7 @@
 #include "smallint.h"
 #include "objgenerator.h"
 #include "lexer.h"
+#include "stackctrl.h"
 
 #if 0 // print debugging info
 #define DEBUG_PRINT (1)
@@ -69,6 +70,8 @@
 };
 
 void mp_init(void) {
+    stack_ctrl_init();
+
     // call port specific initialization if any
 #ifdef MICROPY_PORT_INIT_FUNC
     MICROPY_PORT_INIT_FUNC;