Nothing special is needed to enable the serial console. In fact I'm rather surprised you end up with a login prompt, out-of-the-box LS-UART1 is configured to log in automatically.
Normal behavior is for kernel messages (with timestamps) to pop out for 8-10 seconds, followed by the systemd service start messages (and occasional burst of kernel messages) before finally we get the automatic login messages:
[ OK ] Reached target Graphical Interface.
Starting Update UTMP about System Runlevel Changes...
[ OK ] Started Update UTMP about System Runlevel Changes.
Debian GNU/Linux 8 linaro-alip ttyMSM0
linaro-alip login: root (automatic login)
Last login: Tue Dec 6 08:20:48 UTC 2016 on ttyMSM0
Linux linaro-alip 4.4.23-linaro-lt-qcom #1 SMP PREEMPT Thu Oct 6 08:25:14 UTC 2016 aarch64
The programs included with the Debian GNU/Linux system are free software;
the exact distribution terms for each program are described in the
individual files in /usr/share/doc/*/copyright.
Debian GNU/Linux comes with ABSOLUTELY NO WARRANTY, to the extent
permitted by applicable law.
It should definitely be possible to type into that root prompt in your terminal emulator...