Set Renode target to be no-gui

When building Renode from source, set --no-gui flag so we don't need
the deprecated gtk-sharp2 Debian package (removed from Debian Testing)

Change-Id: I844694764f82b9162d4fba58b1bbc586e40339df

platforms/nexus/renode.mk

diff --git a/platforms/nexus/renode.mk b/platforms/nexus/renode.mk
index 4bd84eb..7100793 100644
--- a/platforms/nexus/renode.mk
+++ b/platforms/nexus/renode.mk
@@ -25,7 +25,7 @@
 
 $(RENODE_BIN): | $(RENODE_SRC_DIR) $(RENODE_OUT_DIR)
 	cd $(RENODE_SRC_DIR); unset PLATFORM; \
-		./build.sh -d -o $(RENODE_OUT_DIR)
+		./build.sh --no-gui -d -o $(RENODE_OUT_DIR)
 	echo -e "built_from_src\ncommit_sha: $(RENODE_COMMIT)\n" > $(RENODE_OUT_DIR)/tag
 
 ## Builds the Renode system simulator

platforms/shodan/renode.mk

diff --git a/platforms/shodan/renode.mk b/platforms/shodan/renode.mk
index 4bd84eb..7100793 100644
--- a/platforms/shodan/renode.mk
+++ b/platforms/shodan/renode.mk
@@ -25,7 +25,7 @@
 
 $(RENODE_BIN): | $(RENODE_SRC_DIR) $(RENODE_OUT_DIR)
 	cd $(RENODE_SRC_DIR); unset PLATFORM; \
-		./build.sh -d -o $(RENODE_OUT_DIR)
+		./build.sh --no-gui -d -o $(RENODE_OUT_DIR)
 	echo -e "built_from_src\ncommit_sha: $(RENODE_COMMIT)\n" > $(RENODE_OUT_DIR)/tag
 
 ## Builds the Renode system simulator