It is possible to have an out-of-tree build of parts of the crazyflie firmware. This enables developers to work on elements without worrrying about merging it with the full code base.
Technically the app layer is an example of an out of tree build. Follow the app layer intructions for this.
In a seperate folder make a Makefile which contain the following content:
CFLAGS += -DOOT_ESTIMATOR VPATH += src/ PROJ_OBJ += estimator_out_of_tree.o CRAZYFLIE_BASE=[LOCATION OF THE CRAZYFLIE FIRMWARE] include $(CRAZYFLIE_BASE)/Makefile
in [your_estimator_out_of_tree].c in the src folder you will just need to make sure that the following functions are filled:
init = estimatorOutOfTreeInit
test = estimatorOutOfTreeTest
update = estimatorOutOfTree
name = "OOT
Not yet implemented. Please request this feature in the crazyflie-firmware issue list