Makefile.include: set default PROGRAMMER value when flahsing on IoT-LAB

This will avoid to load edbg logic and especially adds it to the list of FLASHDEPS: it won't be unnecessarily built when flashing a samr21-xpro/arduino-zero board on IoT-LAB

Makefile.include

@@ -106,6 +106,13 @@ OS := $(shell uname)
 # set python path, e.g. for tests
 PYTHONPATH := $(RIOTBASE)/dist/pythonlibs/:$(PYTHONPATH)
 
+# set default PROGRAMMER value if flashing a board on IoT-LAB
+# This must be done before parsing the Makefile.include of the board and cannot
+# be done in makefile.iotlab.single.inc.mk which is included after.
+ifneq (,$(IOTLAB_NODE))
+  PROGRAMMER ?= iotlab
+endif
+
 # Include Docker settings near the top because we need to build the environment
 # command line before some of the variable origins are overwritten below when
 # using abspath, strip etc.