try tweaking the retroarch.cfg file. Its location is:
There should be some text in the file like:
input_player1_a = x input_player1_b = z input_player1_y = a input_player1_x = s input_player1_start = enter input_player1_select = rshift input_player1_l = q input_player1_r = w input_player1_left = left input_player1_right = right input_player1_up = up input_player1_down = down input_autodetect_enable = true
I like to remote in from a computer using a free program called WinSCP.
You will need to use ROOT as the username and raspberry as the password.