Booting from a fresh copy of Core .iso (or CorePlus, version 12.0, now current for download) picking the default boot option (e.g. 'Boot Core Plus with FLWM topside') from the menu continues on to boot to command line with the following error:
-sh: /bb/tty: not found
sh: tty: unknown operand
This prevents it to boot automatically into the X (FLTK), which then needs a manual 'startx'.
On closer inspection, the problem stems from TERMTYPE=`/bb/tty` entry in .profile file.
The /bb path indeed does not exist any more in Core 12.0 (unlike in older versions). However, it may not be needed, as the busybox commands can be resolved directly (the busybox commands are now linked at '/usr/bin', see 'ls -al /usr/bin/tty').
So, the way to fix this problem is either to patch it in .profile to use '/usr/bin/tty' OR symlink /bb to /usr/bin.