It appears that Xfbdev.core is not installing the file "/etc/sysconfig/Xserver".
If I remaster Micro Core 2.6.1 with Xlibs.core, Xprogs.core, flwm, and Xfbdev.core in /opt/tce and if I boot using "microcore vga=788", then Micro Core boots to the command prompt instead of X. If I execute "startx" (with or without executing "xsetup.sh"), there is an error message about not being able to open the file "/etc/sysconfig/Xserver".
If I add the file "/etc/sysconfig/Xserver" with the text "Xfbdev" to the remaster, Micro Core boots successfully into X.