home: Allow for builds without a home partition

This should allow platforms that currently do not have support for an extra home
partition to still be buildable. Longer term, we should consider how to solve
this problem for those systems, as users will not have space to use for
persistence between operating system flashes.

Change-Id: I78d6c1d9d7b707c57e0bbf5db3d31629f88760f1
diff --git a/home.mk b/home.mk
index c75e542..c2af8ca 100644
--- a/home.mk
+++ b/home.mk
@@ -18,6 +18,7 @@
 include $(ROOTDIR)/build/preamble.mk
+ifneq ($(HOME_SIZE_MB),)
 HOME_RAW_IMG := $(PRODUCT_OUT)/obj/HOME/home.raw.img
 HOME_IMG := $(PRODUCT_OUT)/home.img
@@ -48,3 +49,4 @@
 	@echo "home - creates the home partition image"
 .PHONY:: home home_raw