Start to test new function in AVRUB

[管理员]

I have add a special function in avrub: when enter bootloader, set
flag in eeprom; after update successfully, clear flag. If update fail,
it will not leave bootloader mode, it will enter bootloader again.
Only when update successfully, it will jump to user's application.
This function is useful when you use bootloader remotely. I call this
function as 'safe update mode'.

I have start to test this function, and it will be available soon.