Add the option to reserve headroom for partition images.
This is useful for devices with low disk space with different
build variants.

Bug: 37469715
Test: Regular image builds successfully, errors occur when
the headroom size is greater than available partition space.

Change-Id: I526cdd0f84981bbd16e3afcfe1cd7fc43dce98ef
diff --git a/core/product.mk b/core/product.mk
index 1e5a30e..7742cc3 100644
--- a/core/product.mk
+++ b/core/product.mk
@@ -131,6 +131,7 @@
     VENDOR_EXCEPTION_PATHS \
     PRODUCT_ART_USE_READ_BARRIER \
     PRODUCT_IOT \
+    PRODUCT_SYSTEM_HEADROOM \