android源码核心变量大都在build/core/envsetup.mk中建立
在该文件中，可以找到
TARGET_OUT := $(PRODUCT_OUT)/$(TARGET_COPY_OUT_SYSTEM)
......
TARGET_COPY_OUT_SYSTEM := system
PRODUCT_OUT := $(TARGET_PRODUCT_OUT_ROOT)/$(TARGET_DEVICE)
TARGET_PRODUCT_OUT_ROOT := $(TARGET_OUT_ROOT)/product
TARGET_OUT_ROOT_release := $(OUT_DIR)/target
TARGET_OUT_ROOT_debug := $(DEBUG_OUT_DIR)/target
TARGET_OUT_ROOT := $(TARGET_OUT_ROOT_$(TARGET_BUILD_TYPE))
ifeq (,$(strip $(OUT_DIR)))
ifeq (,$(strip $(OUT_DIR_COMMON_BASE)))
OUT_DIR := $(TOPDIR)out
else
OUT_DIR := $(OUT_DIR_COMMON_BASE)/$(notdir $(PWD))
endif
endif
DEBUG_OUT_DIR := $(OUT_DIR)/debug
# Variables we check:
# HOST_BUILD_TYPE = { release debug }
# TARGET_BUILD_TYPE = { release debug }
# and we output a bunch of variables, see the case statement at
# the bottom for the full list
# OUT_DIR is also set to "out" if it's not already set.
# this allows you to set it to somewhere else if you like
# SCAN_EXCLUDE_DIRS is an optional, whitespace separated list of
# directories that will also be excluded from full checkout tree
# searches for source or make files, in addition to OUT_DIR.
# This can be useful if you set OUT_DIR to be a different directory
# than other outputs of your build syst
所以TARGET_OUT=out/target/product/xxx/system