Should provide option to leave /lib/modules and /lib/firmware out of the squashfs build

Binary package hint: livecd-rootfs

In ubuntu mobile builds we plan to have install options like the following:

if $squashfs_content_size >= $target_device_size -> copy the squashfs onto the disk and set up an overlay FS.
if $squashfs_content_size < $target_device_size -> copy the content of the squashfs onto the disk as we do now with ubiquity.

In the first case having the /lib/modules and /lib/firmware content inside the squashfs wastes a lot of space and makes the squashfs a lot bigger than needed.
It would be nice to have an option to livecd-rootfs that rolls an additional squashed fs with the installed package content of linux-image-* to be merged by casper for booting the install media, but put the original linux-image-*.deb on the install media as well and install this into the copy-on-write merged target device. This way the contents of the linux-image* package wont block space inside the squashfs and it will be possible to remove/replace unwanted old kernels on upgrades to save space.