build: allow refreshmod with external out directory

Get a relative path to out by using $(get_build_var PRODUCT_OUT)
instead of hardcoding it to out/target/product/$TARGET_DEVICE.
That will correctly return the path to the out directory
when setting an external OUT_DIR_COMMON_BASE.

Change-Id: I8a990b710222bc72755c6b6b88fd0c9e80711e14