android: support generating ext4 partition images

Defer generation to libext4_utils, built using the existing sources in
the same style as libgtest.  With the emulator now generating valid ext4
images, re-enable support for ext4 cache partitions.

Bug: 13115061

Change-Id: I8770f2ddbc95f6beb4ca9655ebf9d8f8a7bdd1b0
Signed-off-by: Greg Hackmann <ghackmann@google.com>
7 files changed