Redesign make_ext4fs to incrementally generate ext4 images

Allows passing a blk mapping file through -Z which preserves the
location of those mapping in existing files

Internal Design Doc: go/incremental-ext4
BUG: 26839493
Change-Id: I447873bf65ae66dab9923e5a11c497f0722750df
Signed-off-by: Mohamad Ayyash <mkayyash@google.com>
8 files changed