Generate a primary boot image for testing.

This CL implements a simple commandline tool to generate a primary boot
image on device for testing, and invoke it in a `target_preparer`. See
the bug for why this is done.

Bug: 216467764
Test: All `art_standalone_*_tests`
Test: ArtGtestsTargetChroot
Test: Steps in art/test/README.chroot.md
Change-Id: I412357bbe7ea9ed73b26a0b2631b415ad025fceb
24 files changed