Tidy up some configuration options.

MP_ALLOC_* -> MICROPY_ALLOC_*
MICROPY_PATH_MAX -> MICROPY_ALLOC_PATH_MAX
MICROPY_ENABLE_REPL_HELPERS -> MICROPY_HELPER_REPL
MICROPY_ENABLE_LEXER_UNIX -> MICROPY_HELPER_LEXER_UNIX
MICROPY_EXTRA_* -> MICROPY_PORT_*

See issue #35.
diff --git a/py/vm.c b/py/vm.c
index 123f520..ee2ec23 100644
--- a/py/vm.c
+++ b/py/vm.c
@@ -222,7 +222,7 @@
                                          mp_obj_t *fastn, mp_obj_t **sp_in_out,
                                          mp_exc_stack_t *exc_stack, mp_exc_stack_t **exc_sp_in_out,
                                          volatile mp_obj_t inject_exc) {
-#if MICROPY_USE_COMPUTED_GOTO
+#if MICROPY_OPT_COMPUTED_GOTO
     #include "vmentrytable.h"
     #define DISPATCH() do { \
         TRACE(ip); \
@@ -276,7 +276,7 @@
             // loop to execute byte code
             for (;;) {
 dispatch_loop:
-#if MICROPY_USE_COMPUTED_GOTO
+#if MICROPY_OPT_COMPUTED_GOTO
                 DISPATCH();
 #else
                 TRACE(ip);
@@ -982,7 +982,7 @@
                     fastn[0] = obj1;
                     return MP_VM_RETURN_EXCEPTION;
 
-#if !MICROPY_USE_COMPUTED_GOTO
+#if !MICROPY_OPT_COMPUTED_GOTO
                 } // switch
 #endif
             } // for loop