Build errors and warnings

[aletar89]

The makefile crashes upon build in Vitis2020, same error as described here

The solution is replacing
OUTS = *.o
in the Makefile with
OBJECTS = $(addsuffix .o, $(basename $(wildcard *.c)))
ASSEMBLY_OBJECTS = $(addsuffix .o, $(basename $(wildcard *.S)))

Also, the code of WishboneAXI_selftest.c generates two warning that can be easily cleared.