Makefile: Use (again) pre-built crosvm.ext4

Use the provided pre-built crosvm.ext4 if the user has not supplied a
CROSVM_TAR_GZ. This avoids the need for triggering a re-build of the
image from an archive as that would rely on a mkfs flag not supported by
some build servers.

Change-Id: Iab0c0272e03c99606b6d7f741c1f7a670363c096
1 file changed
tree: 088906a23086507f8a8bcf86723d3fe9602968df
  1. aarch64/
  2. crosvm/
  3. cuttlefish/
  4. kvm-unit-tests/
  5. common.inc
  6. Makefile