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

This is a manual cherry-pick from aosp/master.

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

Merged-In: I526cdd0f84981bbd16e3afcfe1cd7fc43dce98ef

Change-Id: I840d212bb46612074c43b39e94546901f60e4b4c
diff --git a/core/Makefile b/core/Makefile
index b6a704a..e346dff 100644
--- a/core/Makefile
+++ b/core/Makefile
@@ -961,6 +961,7 @@
 $(if $(BOARD_SYSTEMIMAGE_SQUASHFS_BLOCK_SIZE),$(hide) echo "system_squashfs_block_size=$(BOARD_SYSTEMIMAGE_SQUASHFS_BLOCK_SIZE)" >> $(1))
 $(if $(BOARD_SYSTEMIMAGE_SQUASHFS_DISABLE_4K_ALIGN),$(hide) echo "system_squashfs_disable_4k_align=$(BOARD_SYSTEMIMAGE_SQUASHFS_DISABLE_4K_ALIGN)" >> $(1))
 $(if $(PRODUCTS.$(INTERNAL_PRODUCT).PRODUCT_SYSTEM_BASE_FS_PATH),$(hide) echo "system_base_fs_file=$(PRODUCTS.$(INTERNAL_PRODUCT).PRODUCT_SYSTEM_BASE_FS_PATH)" >> $(1))
+$(if $(PRODUCTS.$(INTERNAL_PRODUCT).PRODUCT_SYSTEM_HEADROOM),$(hide) echo "system_headroom=$(PRODUCTS.$(INTERNAL_PRODUCT).PRODUCT_SYSTEM_HEADROOM)" >> $(1))
 $(if $(BOARD_USERDATAIMAGE_FILE_SYSTEM_TYPE),$(hide) echo "userdata_fs_type=$(BOARD_USERDATAIMAGE_FILE_SYSTEM_TYPE)" >> $(1))
 $(if $(BOARD_USERDATAIMAGE_PARTITION_SIZE),$(hide) echo "userdata_size=$(BOARD_USERDATAIMAGE_PARTITION_SIZE)" >> $(1))
 $(if $(BOARD_FLASH_LOGICAL_BLOCK_SIZE), $(hide) echo "flash_logical_block_size=$(BOARD_FLASH_LOGICAL_BLOCK_SIZE)" >> $(1))