switch from genext2fs to mkfs.ext4

Problems with genext2fs:

 - slow O(n^2) runtime
 - need to upgrade ext2 filesystems to ext4 on first boot
 - inode size of 128 disallows timestamps beyond 2038 or sub-second
   precision timestamps

Problems with mkfs.ext4:

 - not bit-by-bit reproducible across multiple invocations

The problems with mkfs.ext4 are solved once this PR is merged:

https://github.com/tytso/e2fsprogs/pull/118