Since generic images have been split to their own
Makefile boards are showing up twice in menuconfig
as $(eval $(call BuildImage)) was not dropped from
the new generic.mk.

Hence $(eval $(call BuildImage)) was being called
twice.

So, lets simply drop it from generic.mk.

Fixes: 378c7ff2

 ("ipq40xx: split generic images into own file")

Signed-off-by: Robert Marko <robert.marko@sartura.hr>