got my usb gamepads working by running
sudo /opt/retropie/emulators/RetroArch/installdir/bin/retroarch-joyconfig -a /opt/retropie/emulators/RetroArch/configs/gamepad.cfg
Also added following lines to cfg file for
input_enable_hotkey_btn = 4
input_exit_emulator_btn = 5
input_save_state_btn = 6
input_load_state_btn = 7
Thus in combination with left trigger , i can exit emulator &save ,load stateswith my gamepad .