Panasonic Eluga U

Hi everyone. I have a Panasonic Eluga U with stock recovery. It only display the Panasonic logo and screen blacks out when the time comes to display Eluga logo. The phone is bricked. I was trying to flash its own system.img which I created before. But it just rebooted before the flash finished. Now I have only stock recovery, download mode and adb sideload available. No idea how to get to Fastboot. Can someone please advise how I can use the system.img file available to unbrick the phone. or to put the phone in fastboot mode. Thanks.