Tweak IS_GLINUX check

Change-Id: Id61b8dec9be66928b85c9de5e4fc754e81ae3fa7
diff --git a/preamble.mk b/preamble.mk
index 746dcd5..4f675f5 100644
--- a/preamble.mk
+++ b/preamble.mk
@@ -15,10 +15,12 @@
 # Preamble. Don't define any targets in this file! This is effectively just a
 # common header where useful global vars go.
 
-IS_GLINUX ?= false
 ifneq (,$(wildcard /etc/dpkg/origins/glinux))
+ifneq (,$(wildcard /google))
 	IS_GLINUX = true
 endif
+endif
+IS_GLINUX ?= false
 
 # Globally useful directories
 TOOLCHAIN := $(ROOTDIR)/toolchains/aarch64-linux-android/bin/aarch64-linux-android-