image_types_phosphor: Add do_generate_ext4_tar()

Add a new do_generate_ext4_tar() function that creates a tarball with
u-boot, kernel, ext4 rootfs and rwfs image files. This tarball is
intended for systems running with a mmc chip.

(From meta-phosphor rev: 02d65b716915f4424feca600f955937e1d16eb1a)

Change-Id: I3545dbb7c36d4ba5a71fb3735324228e8b609a15
Signed-off-by: Adriana Kobylak <anoo@us.ibm.com>
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>