You should attach a debugger and actually try to *devbug* this insetad of guessing (and blaming U-Boot).

Ok, I won't guess anymore ;-) But I don't know how to debug it, I have no debuggers.

One of the things that did change is the mach-types.h file; if your kernel uses some invalid ID this could explain such a hang. But this is wild guessing - please debug the problem.

I verified that entries MACH_TYPE_AT91RM9200 and MACH_TYPE_AT91RM9200DK are the same you found in kernel 2.6.13 and tried replacing MACH_TYPE_AT91RM9200DK MACHINE_START(MACH_TYPE_AT91RM9200DK with MACHINE_START(MACH_TYPE_AT91RM9200 in board-dk.c without success.