Use the new linux-update-symlinks command for symlink updates.
Drop support for minimal_swap, no_symlinks, use_hard_links and *_hook
parameters in /etc/kernel-img.conf.
Thanks to Aurelien Jarno and Santiago Vila for pointing out the
ischroot command and that it's in an essential package (i.e. doesn't
add a dependency).
This seems to have been a transitional feature for the switch
from kernel-image to linux-image. mkvmlinuz still uses it but
no hook scripts appear to depend on it.