I’ve just experienced a very similar issue to this. It was rebooting after approx. 2.5 seconds, after a line which read mmcblk0: p1 p2 p3 p4 < p5 p5 >. I took a look at my PSU, and noticed it was only rated to 9V x 300mA (or “Why is that even in my drawer!”), so was browning out when the Pi started doing anything serious. I replaced the PSU with a 12V x 6A I had knocking around, didn’t even re-etch the MMC, and it now boots fine and reports in.