default.prop

就是在 android/build/core/Makefile:

# -----------------------------------------------------------------
# default.prop
INSTALLED_DEFAULT_PROP_TARGET := $(TARGET_ROOT_OUT)/default.prop
ALL_DEFAULT_INSTALLED_MODULES += $(INSTALLED_DEFAULT_PROP_TARGET)
ADDITIONAL_DEFAULT_PROPERTIES := \
    $(call collapse-pairs, $(ADDITIONAL_DEFAULT_PROPERTIES))
ADDITIONAL_DEFAULT_PROPERTIES += \
    $(call collapse-pairs, $(PRODUCT_DEFAULT_PROPERTY_OVERRIDES))
ADDITIONAL_DEFAULT_PROPERTIES := $(call uniq-pairs-by-first-component, \
    $(ADDITIONAL_DEFAULT_PROPERTIES),=)

$(INSTALLED_DEFAULT_PROP_TARGET):
        @echo Target buildinfo: $@
        @mkdir -p $(dir $@)
        $(hide) echo "#" > $@; \
                echo "# ADDITIONAL_DEFAULT_PROPERTIES" >> $@; \
                echo "#" >> $@;
        $(hide) $(foreach line,$(ADDITIONAL_DEFAULT_PROPERTIES), \
                echo "$(line)" >> $@;)
        build/tools/post_process_props.py $@

所以在所有 *.mk 中，有宣告 ADDITIONAL_DEFAULT_PROPERTY_OVERRIDES 的，都會被加入 default.prop 中。
例如：

android/devices/common/common.mk:
# Set default USB interface
PRODUCT_DEFAULT_PROPERTY_OVERRIDES += \
        persist.sys.usb.config=mass_storage \
        persist.sys.strictmode.disable=1

另外，script 又多加處理某些部份，寫在 build/tools/post_process_props.py:

# Put the modifications that you need to make into the /system/build.prop into this
# function. The prop object has get(name) and put(name,value) methods.
def mangle_default_prop(prop):
  # If ro.debuggable is 1, then enable adb on USB by default
  # (this is for userdebug builds)
  if prop.get("ro.debuggable") == "1":
    val = prop.get("persist.sys.usb.config")
    if val == "":
      val = "adb"
    else:
      val = val + ",adb"
    prop.put("persist.sys.usb.config", val)
  # UsbDeviceManager expects a value here.  If it doesn't get it, it will
  # default to "adb". That might not the right policy there, but it's better
  # to be explicit.
  if not prop.get("persist.sys.usb.config"):
    prop.put("persist.sys.usb.config", "none");

他加上 persist.sys.usb.config adb

所以最後 rootfs/default.prop 就有：

persist.sys.usb.config=mass_storage,adb