maas i386/*18.04* images fail to successfully deploy with maas 3.1

[Sean Feole]

Yesterday some of the i386 deployments for kernel tests failed. There appears to be an issue with the maas images that only affects i386/18.04.

The issue may have existed a lot longer, since we were using some alternative means to deploy i386/18.04 on our test machines.

See the attached logs
I have begun to triage this problem and feel it could be related to the boot-initrd generated for i386/bionic. I will update the bug as I find more information.

The Attached Logs are from a deployment of i386/18.04-hwe-edge

[ 6.667640] Write protecting kernel text and read-only data: 18368k
[ 6.674760] NX-protecting the kernel data: 12532k
[ 6.680638] x86/mm: Checked W+X mappings: passed, no W+X pages found.
[ 6.688010] x86/mm: Checking user space page tables
[ 6.693642] x86/mm: Checked W+X mappings: passed, no W+X pages found.
[ 6.700945] Run /init as init process
[ 6.705199] Failed to execute /init (error -2)
[ 6.710267] Run /sbin/init as init process
[ 6.714961] Run /etc/init as init process
[ 6.719552] Run /bin/init as init process
[ 6.724136] Run /bin/sh as init process
[ 6.728529] Kernel panic - not syncing: No working init found. Try passing init= option to kernel. See Linux Documentation/admin-guide/init.rst for guidance.
[ 6.744519] CPU: 1 PID: 1 Comm: swapper/0 Not tainted 5.4.0-109-generic #123~18.04.1-Ubuntu
[ 6.753976] Hardware name: Dell Inc. PowerEdge R320/0DY523, BIOS 1.2.4 05/11/2012
[ 6.762462] Call Trace:
[ 6.765297] dump_stack+0x60/0x7a
[ 6.769097] ? __noinstr_text_end+0x2a00/0x2a10
[ 6.774257] panic+0xa8/0x26f
[ 6.777669] ? rest_init+0xa0/0xa0
[ 6.781568] kernel_init+0xee/0x100
[ 6.785565] ret_from_fork+0x2e/0x38
[ 6.789696] Kernel Offset: 0x15000000 from 0xc1000000 (relocation range: 0xc0000000-0xf7dfdfff)
[ 6.803177] ---[ end Kernel panic - not syncing: No working init found. Try passing init= option to kernel. See Linux Documentation/admin-guide/init.rst for guidance. ]--

ii maas 1:3.1.0-10901-g.f1f8f1505-0ubuntu1~20.04.1 all "Metal as a Service" is a physical cloud and IPAM
ii maas-cli 1:3.1.0-10901-g.f1f8f1505-0ubuntu1~20.04.1 all MAAS client and command-line interface
ii maas-common 1:3.1.0-10901-g.f1f8f1505-0ubuntu1~20.04.1 all MAAS server common files
ii maas-dhcp 1:3.1.0-10901-g.f1f8f1505-0ubuntu1~20.04.1 all MAAS DHCP server
ii maas-proxy 1:3.1.0-10901-g.f1f8f1505-0ubuntu1~20.04.1 all MAAS Caching Proxy
ii maas-rack-controller 1:3.1.0-10901-g.f1f8f1505-0ubuntu1~20.04.1 all Rack Controller for MAAS
ii maas-region-api 1:3.1.0-10901-g.f1f8f1505-0ubuntu1~20.04.1 all Region controller API service for MAAS
ii maas-region-controller 1:3.1.0-10901-g.f1f8f1505-0ubuntu1~20.04.1 all Region Controller for MAAS
ii python3-django-maas 1:3.1.0-10901-g.f1f8f1505-0ubuntu1~20.04.1 all MAAS server Django web framework (Python 3)
ii python3-maas-client 1:3.1.0-10901-g.f1f8f1505-0ubuntu1~20.04.1 all MAAS python API client (Python 3)
ii python3-maas-provisioningserver 1:3.1.0-10901-g.f1f8f1505-0ubuntu1~20.04.1 all MAAS server provisioning libraries (Python 3)

ubuntu@maas2:~$ dpkg -l | grep curtin
ii curtin-common 21.3-1004-g8c131399-0ubuntu1+307~trunk~ubuntu20.04.1 all Library and tools for curtin installer
ii python3-curtin 21.3-1004-g8c131399-0ubuntu1+307~trunk~ubuntu20.04.1 all Library and tools for curtin installer

$ sudo apt-cache policy curtin-common
curtin-common:
  Installed: 21.3-1004-g8c131399-0ubuntu1+307~trunk~ubuntu20.04.1
  Candidate: 21.3-1004-g8c131399-0ubuntu1+307~trunk~ubuntu20.04.1
  Version table:
 *** 21.3-1004-g8c131399-0ubuntu1+307~trunk~ubuntu20.04.1 500
        500 http://ppa.launchpad.net/curtin-dev/daily/ubuntu focal/main amd64 Packages
        100 /var/lib/dpkg/status
     21.3-0ubuntu1~20.04.1 500
        500 http://ppa.launchpad.net/maas/3.1/ubuntu focal/main amd64 Packages
        500 http://ppa.launchpad.net/maas/3.2/ubuntu focal/main amd64 Packages
     21.2-0ubuntu1~20.04.1 500
        500 http://ppa.launchpad.net/maas/2.9/ubuntu focal/main amd64 Packages
     20.1-2-g42a9667f-0ubuntu1~20.04.1 500
        500 http://archive.ubuntu.com/ubuntu focal-updates/main amd64 Packages
     19.3-27-g437caaa9-0ubuntu1 500
        500 http://archive.ubuntu.com/ubuntu focal/main amd64 Packages