[U-Boot] secure embedded linux system

hello,,I am from India.I am electronics hobbyist.currently I am using beaglebone black in my project.and I am afraid of security of linux systemits quite easy to copy or modify data from linux system if it has physical access.lets say ,I have BBB , and I boot it from external device(like external mmc ). and mount internal emmc then modify shadow file. and then boot from internal Emmc. now I will get root access , and I can modify anything on the system. sir, I want to secure embedded linux system(BBB). Kindly help on this..how can I secure data on beaglebone black..?? without entering password during boot process..(in case of encrypted rootfs)

thanks in advance.. Best Regards,,Mahendracontact number:+91-9033544852