build: archive make install-ext-deps build logs in ci

- also add build/external artifacts to .gitignore file

Type: make

Change-Id: I9c8f4a312bbfddfa8024531e7246518c8c3589a4
Signed-off-by: Dave Wallace <dwallacelf@gmail.com>
diff --git a/build/external/Makefile b/build/external/Makefile
index 6b5f905..ce57c79 100644
--- a/build/external/Makefile
+++ b/build/external/Makefile
@@ -30,6 +30,12 @@
 
 B := $(BUILD_DIR)
 I := $(INSTALL_DIR)
+ifeq ($(WORKSPACE),)
+L := $(B)
+else
+L := $(WORKSPACE)/archives/vpp-ext-deps
+$(shell rm -rf $(L) && mkdir -p $(L))
+endif
 
 ifneq ($(shell which cmake3),)
 CMAKE?=cmake3