I took the filesystem from here:Ubuntu Core 14.04 LTS (Trusty Tahr)
and put int onto a Ext3 formatted USB hard drive.
I copied over the kernel modules.
I put this into the /etc/fstab:
proc /proc proc defaults 0 0
devpts /dev/pts devpts mode=0620,gid=5 0 0
usbdevfs /proc/bus/usb usbdevfs noauto 0 0
tmpfs /run tmpfs mode=0755,nodev,nosuid,strictatime 0 0
tmpfs /var/volatile tmpfs defaults 0 0
And this into /etc/init/ttyAMA0.conf:
# ttyAMA0 - getty
# This service maintains a getty on ttyS0 from the point the system is
# started until it is shut down again.
start on stopped rc or RUNLEVEL=
stop on runlevel [!12345]
exec /sbin/getty -L 115200 ttyAMA0 vt102
But it doesn't boot. I get lots of messages about plymouth and upstart doing things but then it just seems to hang.
If I add init=/bin/bash to the kernel boot command I can get into single user mode.
Anyway, my real question is should I be expecting this to work at all? I just want a more featured filesystem than OpemEmbedded ......
I haven't tried Ubuntu. But, if you are looking for alternatives to the Linaro supplied deliverables...
I did try (briefly) the Debian pre-built rootfses here: https://wiki.debian.org/Arm64Port
These might help you.
I have been using Ubuntu 14.04 from it's beta release with same changes as you have mentioned
for ttyAMA0. but IIRC the release even has those ttyAMA0 changes.
It works fine to me. I was able to boot  with just a change to disable cloud-init:
etc/cloud/cloud.cfg.d/90_dpkg.cfg: datasource_list: [ None ]