I started off a minimal ub-Core-trusty installation with kernel and initrd (recent version of 20150505) plus persistent home and opt on a dedicated ext4-partition. Importing xorg-all, graphics-3.16.6-tinycore and flwm with "-b" flag does boot into an X desktop with full-screen Xterm only. Upon checking, I recognized that '~/.xinitrc' does not exist.
Should it not get created automatically and if so where? It is not in '/etc/skel/' so I presume it should be created from xorg-all. Searching the full '/tmp/tcloop/' for "*init*" does not reveal anything promising.