remember device-specific releasetools extensions in target-files

Store the location of the releasetools extensions in the target-files
zip, and make ota_from_target_files use that stored location by
default (though it can still be overridden with -s if desired).
diff --git a/core/Makefile b/core/Makefile
index db70d2f..e704748 100644
--- a/core/Makefile
+++ b/core/Makefile
@@ -814,6 +814,13 @@
 
 $(BUILT_TARGET_FILES_PACKAGE): PRIVATE_RECOVERY_API_VERSION := $(RECOVERY_API_VERSION)
 
+ifeq ($(TARGET_RELEASETOOLS_EXTENSIONS),)
+# default to common dir for device vendor
+$(BUILT_TARGET_FILES_PACKAGE): tool_extensions := $(TARGET_DEVICE_DIR)/../common
+else
+$(BUILT_TARGET_FILES_PACKAGE): tool_extensions := $(TARGET_RELEASETOOLS_EXTENSIONS)
+endif
+
 # Depending on the various images guarantees that the underlying
 # directories are up-to-date.
 $(BUILT_TARGET_FILES_PACKAGE): \
@@ -887,6 +894,7 @@
 	$(hide) echo "recovery $(call image-size-from-data-size,$(BOARD_RECOVERYIMAGE_PARTITION_SIZE))" >> $(zip_root)/META/imagesizes.txt
 	$(hide) echo "system $(call image-size-from-data-size,$(BOARD_SYSTEMIMAGE_PARTITION_SIZE))" >> $(zip_root)/META/imagesizes.txt
 	$(hide) echo "userdata $(call image-size-from-data-size,$(BOARD_USERDATAIMAGE_PARTITION_SIZE))" >> $(zip_root)/META/imagesizes.txt
+	$(hide) echo "$(tool_extensions)" > $(zip_root)/META/tool-extensions.txt
 	@# Zip everything up, preserving symlinks
 	$(hide) (cd $(zip_root) && zip -qry ../$(notdir $@) .)
 
@@ -909,13 +917,6 @@
 
 $(INTERNAL_OTA_PACKAGE_TARGET): KEY_CERT_PAIR := $(DEFAULT_KEY_CERT_PAIR)
 
-ifeq ($(TARGET_RELEASETOOLS_EXTENSIONS),)
-# default to common dir for device vendor
-$(INTERNAL_OTA_PACKAGE_TARGET): extensions := $(TARGET_DEVICE_DIR)/../common
-else
-$(INTERNAL_OTA_PACKAGE_TARGET): extensions := $(TARGET_RELEASETOOLS_EXTENSIONS)
-endif
-
 ifeq ($(TARGET_OTA_SCRIPT_MODE),)
 # default to "auto"
 $(INTERNAL_OTA_PACKAGE_TARGET): scriptmode := auto
@@ -926,7 +927,7 @@
 $(INTERNAL_OTA_PACKAGE_TARGET): $(BUILT_TARGET_FILES_PACKAGE) otatools
 	@echo "Package OTA: $@"
 	$(hide) ./build/tools/releasetools/ota_from_target_files \
-	   -s $(extensions) -m $(scriptmode) \
+	   -m $(scriptmode) \
 	   -p $(HOST_OUT) \
            -k $(KEY_CERT_PAIR) \
            $(BUILT_TARGET_FILES_PACKAGE) $@