adding avrdude dump flash support

[SuperUserNameMan]

Hello @brother-yan ,

I forked your project to add support for the dump flash command of AVRdude in terminal mode.

Unfortunately, I made too much changes to the code at once (reindentation and cleanup) and I don't remember how to use GIT to make pull request to you (i just copy paste my code to github).

Though, if you want to add this functionality to your version, you'll find it into the universal() function of the .ino file : https://github.com/SuperUserNameMan/LGTISP

( My fork also incorporate an update from an other fork mentioned in the comments, and I've just noticed that I forgot to reincorporate your case 'z': ... SWD_ReadGUID(); into mine. )

If you want to join discussion about how to improve LarduinoISP / LGTISP, you can join us here : https://github.com/dbuezas/lgt8fx-forum/issues/1