diff --git a/makefiles/murdock.inc.mk b/makefiles/murdock.inc.mk
index f34d8ea15b..945802f701 100644
--- a/makefiles/murdock.inc.mk
+++ b/makefiles/murdock.inc.mk
@@ -2,8 +2,8 @@
 # This file contains helper targets used by the CI.
 #
 
-# (HACK) get actual flash binary from FFLAGS.
-FLASHFILE:=$(filter $(HEXFILE) $(ELFFILE:.elf=.bin) $(ELFFILE),$(FFLAGS))
+# (HACK) get actual flash binary from FFLAGS if not defined.
+FLASHFILE ?= $(filter $(HEXFILE) $(ELFFILE:.elf=.bin) $(ELFFILE),$(FFLAGS))
 
 #
 # This target will run "make test" on the CI cluster.