Summary:
The initramfs job fails to build an initrd.img for two reasons:
1. No /boot/vmlinuz-$version
2. No /boot/initrd.img-$version
(1) is due to "contextualprocess@before_bootloader_mkdirs" needing to
be executed before "initramfs" because it copies in the kernel image
from /cdrom/casper/vmlinuz.
(2) is due to a bug in update-initramfs::get_sorted_versions() because
it expects there to be an existing initrd.img-$version despite the fact
the "initramfs" module is calling it with the -c (create) option, not
-u (update).
Closes (LP: #1829805)
Test Plan: Edited config in Live Environment and successfully rebooted into the installed OS.
Reviewers: wxl, tsimonq2
Reviewed By: wxl
Differential Revision: https://phab.lubuntu.me/D15
