vndk-def: Add `--aosp-system` option

This commit adds `--aosp-system` option to simplify the command line
usages.  The user can specify the aosp image system directory directly
and skip the `create-generic-ref` subcommand.

Test: Compare the output with `--load-generic-refs` and `--aosp-system`.
Change-Id: Ibb76bd59ea504808545775562feefcd8a95ee267