Using the raspberian OS, I set mine to automatically login by editing
/etc/inittabComment this line:
1:2345:respawn:/sbin/getty 115200 tty1
Add this line below it (here, pi is the user)
1:2345:respawn:/bin/login -f pi tty1 </dev/tty1 >/dev/tty1 2>&1
Now that you're automatically logged in, I'm guessing that your rc.local file should now start the mining process.
If you have security concerns, do NOT do what I just posted