Comment 2 for bug 2051004

@gustavo, you have more or less answered to yourself here :)
I double-checked, there is no "hpsa.ko" in the initrd, which means you have no storage to start with, that's the reason your "mount initial filesystems" is failing.
I had experienced an almost identical problem in the past with emmc on x86 when emmc module was missing in the initrd and boot was failing in the same way.

We can build a custom kernel snap with "hpsa.ko" included in the initrd to confirm this.

As a proper resolution, you would need to request hpsa.ko(it will also pull in scsi_transport_sas.ko) being added to the initrd from Ubuntu Core team. Size increase should be negligible (<100kB).