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>

1 files changed, 3 insertions, 0 deletions

diff --git a/rules.mak b/rules.mak
index 04c7f74d07..bbb2667928 100644
--- a/rules.mak
+++ b/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