My Raspbian Pi crashes to a Kernel panic when I start it up, and it suggested somewhere that an invalid cmdline.txt can cause this. What is in the cmdline.txt by default?
My (unaltered) PI2B has a /boot/cmdline.txt of
dwc_otg.lpm_enable=0 console=ttyAMA0,115200 console=tty1 root=/dev/mmcblk0p2 rootfstype=ext4 elevator=deadline rootwait
There should only be one uncommented line. It is not like /boot/config.txt where you have entries on multiple lines. Line starting with # are ignored so if you want to change the line I suggest you leave the original with a # at the start.