Add option to include icons in exercises

Change-Id: Iaaf5ed909c931a57bd036d530816bdf158638d1f
Signed-off-by: Sasa Zivkov <sasa.zivkov@sap.com>
diff --git a/exercises/Makefile-common b/exercises/Makefile-common
index 24911be..773084e 100644
--- a/exercises/Makefile-common
+++ b/exercises/Makefile-common
@@ -1,5 +1,5 @@
 ASCIIDOC ?= asciidoc
-ASCIIDOC_EXTRA ?=
+ASCIIDOC_EXTRA ?= -a icons -a iconsdir=/opt/local/etc/asciidoc/images/icons
 
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