Write AB_OTA_POSTINSTALL_CONFIG to META/postinstall_config.txt.

The config file will be passed to delta_generator to add the postinstall
field in the update payload.

Bug: 24537566
Change-Id: I06b8d9d0b2c37f9a3e7a844ac4d9e2eead8cd4d2
TEST: make dist
diff --git a/core/Makefile b/core/Makefile
index 8922f06..c930708 100644
--- a/core/Makefile
+++ b/core/Makefile
@@ -1629,6 +1629,9 @@
 	$(hide) for part in $(AB_OTA_PARTITIONS); do \
 	  echo "$${part}" >> $(zip_root)/META/ab_partitions.txt; \
 	done
+	$(hide) for conf in $(AB_OTA_POSTINSTALL_CONFIG); do \
+	  echo "$${conf}" >> $(zip_root)/META/postinstall_config.txt; \
+	done
 	@# Include the build type in META/misc_info.txt so the server can easily differentiate production builds.
 	$(hide) echo "build_type=$(TARGET_BUILD_VARIANT)" >> $(zip_root)/META/misc_info.txt
 ifdef OSRELEASED_DIRECTORY