isolinux: fix toram=... variable usage within isolinux configs
Fixup for commit
9453222c0, where we introduced usage of
$fixed_squashfs_name. This variable was set up only *after*
adjust_boot_files function got invoked for the isolinux configuration
files. As a result the isolinux configuration files ended up with their
template placeholder settings included ("toram=%SQUASHFS_NAME%").
Fix ordering, but also warn in adjust_boot_files() if the the
$fixed_squashfs_name + $fixed_release_info variables are unset.
Thanks: Chris Hofstaedtler for the bug report
Closes: https://github.com/grml/grml/issues/179