@sano You know what dude? I solved the problem myself :D . You know PPSSPP for Windows for e.g. It's in the same form as the one I mentioned. But now, I want to mess with es_systems.cfg, to run it through the Emulationstation menu. You see, the reason of can only boot one file error, was that in the es_systems.cfg file, there was line like this:
<command>/opt/retropie/emulators/ppsspp/PPSSPPDL</command>
And when I was trying to open .iso file through th ES menu, RetroPie was cofused, whether it should run PPSSPPDL, or .iso file. That was the reason of occuring can only boot one file error message. Coming back to es_systems.cfg, how to edit it in order to run PPSSPPDL, through psp section??? Hardware list, if necessary:
Raspberry Pi 2 Model B
32GB Sandisk SD Card
TP-LINK 150Mbps Wireless Nano USB Adapter (Model TL- WN725N)
Esperanza Bluetooth USB Dongle (model wasn't specified on the box, I'm really sorry...)
PPSSPPDL (non-libretrocore version, but I told you about it before)