Add support for creating ext4 images with mke2fs

We are investigating replacing make_ext4fs with the upstream tool mke2fs.
See b/23686092 for more informations.

To mitigate the trouble that may arise if the new tool behave differently
compared to the old one, there will be a transition period where both mke2fs
and make_ext4fs will be supported.

This patch does 3 things:
  - add the necessary code to use mke2fs to format an ext4 partition;
  - add a dependency to the binary used by vold.

Test: m -j32 with TARGET_USES_MKE2FS={,false,true}
                  TARGET_USERIMAGES_USE_EXT4={,true}

Change-Id: I89222642fe1d11a035155c8224b84b2e3719938b
3 files changed