In bug #1646197 the linux-firmware.postinst was added to call update-initramfs (provided by initramfs-tools. If the initramfs-tools package is not installed the installation of linux-firmware will fail.
Recreate:
sudo apt-get purge --assume-yes '^linux-.*' 'linux-base+' initramfs*
sudo apt-get install --assume-yes linux-generic
Result:
Setting up linux-modules-4.15.0-34-generic (4.15.0-34.37) ...
Setting up linux-headers-4.15.0-34 (4.15.0-34.37) ...
Setting up linux-headers-4.15.0-34-generic (4.15.0-34.37) ...
Setting up initramfs-tools-bin (0.130ubuntu3.3) ...
Setting up linux-firmware (1.173.1) ...
update-initramfs: Generating /boot/initrd.img-4.15.0-1021-kvm
/usr/sbin/mkinitramfs: 66: .: Can't open /etc/initramfs-tools/initramfs.conf
update-initramfs: failed for /boot/initrd.img-4.15.0-1021-kvm with 2.
dpkg: error processing package linux-firmware (--configure):
installed linux-firmware package post-installation script subprocess returned error exit status 2
...
Errors were encountered while processing:
linux-firmware
linux-image-generic
linux-generic
E: Sub-process /usr/bin/dpkg returned an error code (1)
Impact:
Builds of minimized images start with an image that has the linux-kvm kernel but lack initramfs-tools. Derivative builds that require a different kernel, linux-generic for example, will purge the existing kernel and install the correct kernel which could pull in linux-firmware and initramfs-tools. In that case the builds fail unless initramfs-tools is installed prior to kernel installation as a workaround for this missing package dependency.
This has been recreated on cosmic and bionic, a fix is desired on each. (I was unable to recreate on Xenial but that package lacks the dependency as well).
script output from a recreate on bionic demonstrating the failure