Well I don't think that the uEnv.txt is a problem in your case.
But this is mine:
uenvcmd=setenv os_type linux;
bootargs=earlyprintk clk_ignore_unused selinux=0 scandelay root=/dev/mmcblk0p2 roofstype=ext4 rootwait
This rebooting you have is not "normal". It remembers me some problems with powersupply or powersupply-cable.
You have no problem with the ready images like debian?