Bug#594968: Acknowledgement (Support mksquashfs -e by prepending (not appending) to MKSQUASHFS_OPTIONS.)
severity 594968 wishlist
Just after posting this, I found config/binary_rootfs/excludes support
coded in binary_chroot, which basically calls rm -rf on a bunch of
globs prior to the mksquashfs run.
It's not my preferred way of doing it (rm -r is O(1), though obviously
it also works for squashfs-less builds), but it DOES work, so I've
downgraded this ticket to "wishlist".