I want the game icon part xd.
If that's all you want, I'd say it's REALLY
not going to be worth it.
You'll have to reverse engineer the updater app, and modify the spoofed chart header.
Keep in mind, that that spoofed header is the only reason it boots on a dsi or 3ds...
I don't know how much of that data is actually checked by the 3ds, but modding that could make your flash cart unusable.
If you're actually serious
about doing this, (changing the icon, and autobooting the game) then you could try to get in contact with the guys who worked on the (single system) ntr boot installer.
But if you can't even fully understand this
, You really shouldn't even bother.
Not trying to be mean, you just gotta be realistic,
Are you actually willing to spend a few weeks learning C++,
Then maybe a few months
to get to the point where you can understand, and implement the existing code,
All for just the possibility
that you can change the flashcart icon?
If not, sorry, but I guess your quest ends here.