tests/docker/Makefile.include: handle empty TARGET_LIST

If the user doesn't specify a TARGET_LIST they get the current
configuration but with spaces and hilarity ensues. This adds some make
magic to turn the TARGET_LIST back into a comma separated list.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>

Message-Id: <20180521103504.26432-1-alex.bennee@linaro.org>
Signed-off-by: Fam Zheng <famz@redhat.com>

rules.mak

@@ -1,4 +1,7 @@
 
+# These are used when we want to do substitutions without confusing Make
+NULL  :=
+SPACE := $(NULL) #
 COMMA := ,
 
 # Don't use implicit rules or variables

tests/docker/Makefile.include

@@ -140,7 +140,7 @@ docker-run: docker-qemu-src
 			$(if $V,,--rm) 					\
 			$(if $(DEBUG),-ti,)				\
 			$(if $(NETWORK),$(if $(subst $(NETWORK),,1),--net=$(NETWORK)),--net=none) \
-			-e TARGET_LIST=$(TARGET_LIST) 			\
+			-e TARGET_LIST=$(subst $(SPACE),$(COMMA),$(TARGET_LIST))	\
 			-e EXTRA_CONFIGURE_OPTS="$(EXTRA_CONFIGURE_OPTS)" \
 			-e V=$V -e J=$J -e DEBUG=$(DEBUG)		\
 			-e SHOW_ENV=$(SHOW_ENV) 			\