build.sh: Add ext4.img to recognized image artifacts (v5.1.1)

:Release Notes:
Archive also ext4.img format when available.

:Detailed Notes:

:Testing Performed:
Only build tested.

:QA Notes:
No change to image.

:Issues Addressed:
[ES-1181] Add support to save ext4.img files in builds

Open-webOS-DCO-1.0-Signed-off-by: Martin Jansa <martin.jansa@lge.com>
Change-Id: I83e29fcd08be829d0d29dc3655a8e4122c73f149
Reviewed-on: https://g2g.palm.com/6382
Reviewed-by: DCO Verification
Reviewed-by: Susan Montooth <susan.montooth@lge.com>
Tested-by: Susan Montooth <susan.montooth@lge.com>