Simulator: add the argument to keep the updated images

Add the command line option to select the work directory and save the
updated image files. Because some people might have interested in
getting updated images from an ota file.

Also, fix a minor issue that the destination of package_extract_file
needs to be updated if it's a block device. Otherwise, an unintended
file may be extracted in the callers' directory.

Test: run simulation, run unit tests

Change-Id: Ic6a7db0580bc1748d6e080102e4654da4e41fd8c