fpga: Allow user to override MAX_TARGET

tests/Makefile.am

@@ -1111,7 +1111,7 @@ endif
 
 # Maxeler compiler
 if STARPU_USE_FPGA
-MAX_TARGET = DFE_SIM
+MAX_TARGET ?= DFE_SIM
 MAX_DFE = MAX5C_$(MAX_TARGET)
 
 MAXJ=$(wildcard fpga/*.maxj)