Add build rules for crosvm.img prebuilt

Check in download script from Cuttlefish GitHub and add a build rule
which uses it to download Cuttlefish host tools (includes CrosVM) and
packages it into a ext4 image in prebuilts/.

Change-Id: Ia3b9bc59ea081a2b03af262204ae577096dc236e
2 files changed
tree: 00abdd1566821f2f08da199e8040e133d7e5ee2f
  1. aarch64/
  2. cuttlefish/
  3. kvm-unit-tests/
  4. common.inc
  5. Makefile