Add INIT_EXPORT_EX macro define

This macro will help the user to organize the start sequence of board/device/application initialization functions.
We add three per-define macros:
* INIT_BOARD_EXPORT_EX(fn, index)
* INIT_DEVICE_EXPORT_EX(fn, index)
* INIT_APP_EXPORT_EX(fn, index)

The additional parameter index should be a number. When using the micro INIT_XXXX_EXPORT_EX(fn, index), index will be converted to a string to identify the sequence.
e.g If we have 5 init functions and set the index as 0,1,01,10,02. Then the sequence should be 0->01->02->1->10