Hi Tobias,
__ghs_board_devices_init_AFTER_main is defined in
<zip file>\AN4830_rev1_SW\MPC5748G_rev<revision>_silicon\tgt\libboardinit\mpc5748g_rev1.ppc
Normally, compiler should find it, since demo projects contain and build
tgt\resources.gpj
which builds libboardinit.gpj, containing mentioned file.
BR,
Alex