One more try at IS_EXTERNAL

Change-Id: Ie004e029e8c69d2d52a3288db72ba2eebb3cc055
diff --git a/preamble.mk b/preamble.mk
index 1569c66..8b29f49 100644
--- a/preamble.mk
+++ b/preamble.mk
@@ -17,14 +17,14 @@
 
 LOG := @$(ROOTDIR)/build/log.sh
 
-ifneq (,$(wildcard /etc/dpkg/origins/glinux))
-ifneq (,$(wildcard /google))
-	IS_EXTERNAL ?= true
+ifeq (,$(wildcard /etc/dpkg/origins/glinux))
+ifeq (,$(wildcard /google))
+  IS_EXTERNAL ?= true
 endif
 endif
 
 FETCH_PACKAGES ?= false
-ifeq ($(IS_EXTERNAL),false)
+ifeq ($(IS_EXTERNAL),)
   PREBUILT_DOCKER_ROOT ?= /google/data/ro/teams/spacepark/enterprise/kokoro/prod/spacepark/enterprise/docker/
   FETCH_PBUILDER_DIRECTORY ?= /google/data/ro/teams/spacepark/enterprise/kokoro/prod/spacepark/enterprise/pbuilder/
   ROOTFS_RAW_CACHE_DIRECTORY ?= /google/data/ro/teams/spacepark/enterprise/kokoro/prod/spacepark/enterprise/rootfs/latest/