isolinux: fix toram=... variable usage within isolinux configs

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