py/modbuiltins: Add additional macro for extending builtins.

Mainly useful for defining additional globals in boards and variants.

py/modbuiltins.c

@@ -775,6 +775,7 @@ STATIC const mp_rom_map_elem_t mp_module_builtins_globals_table[] = {
 
     // Extra builtins as defined by a port
     MICROPY_PORT_BUILTINS
+    MICROPY_PORT_EXTRA_BUILTINS
 };
 
 MP_DEFINE_CONST_DICT(mp_module_builtins_globals, mp_module_builtins_globals_table);

py/mpconfig.h

@@ -1581,6 +1581,12 @@ typedef double mp_float_t;
 #define MICROPY_PORT_BUILTINS
 #endif
 
+// Additional builtin function definitions for extension by command-line, boards or variants.
+// See modbuiltins.c:mp_module_builtins_globals_table for format.
+#ifndef MICROPY_PORT_EXTRA_BUILTINS
+#define MICROPY_PORT_EXTRA_BUILTINS
+#endif
+
 // Additional builtin module definitions - see objmodule.c:mp_builtin_module_table for format.
 #ifndef MICROPY_PORT_BUILTIN_MODULES
 #define MICROPY_PORT_BUILTIN_MODULES