stmhal: Enable new str.splitlines() method.

stmhal/mpconfigport.h

@@ -57,6 +57,7 @@
 #define MICROPY_CAN_OVERRIDE_BUILTINS (1)
 #define MICROPY_PY_FUNCTION_ATTRS   (1)
 #define MICROPY_PY_BUILTINS_STR_UNICODE (1)
+#define MICROPY_PY_BUILTINS_STR_SPLITLINES (1)
 #define MICROPY_PY_BUILTINS_MEMORYVIEW (1)
 #define MICROPY_PY_BUILTINS_FROZENSET (1)
 #define MICROPY_PY_BUILTINS_EXECFILE (1)