isohybrid binary moved to syslinux-utils, update Recommends + error msg
Do our best to make sure aptitude is installed in chroot

There seems to be a change in Debian (jessie) causing aptitude to
no longer be present in debootstrapped systems by default any
longer. By setting $FAI_DEBOOTSTRAP_OPTS accordingly we include
it in its default package selection though. If it still shouldn't
be present we explicitely try to install it through the
updatebase.GRMLBASE hook script.
Update lintian overrides for modified lintian message
Release new version 0.26.0