image_types_phosphor: Add do_generate_rwfs_ext4()

Create a new do_generate_rwfs_ext4() function that generates an empty
ext4 rwfs image file of arbitrary size 1MB. This file is needed to create
a tarball image for code update to use. The size can later be updated,
this commit is intended as a first step for creating a tarball with ext4
image files.

(From meta-phosphor rev: 846990eb90e662176c9e8c395e1093ebf7e6934c)

Change-Id: Idf9cd1dc4f79c2f6cf7f567b8abaf35123a5ca6a
Signed-off-by: Adriana Kobylak <anoo@us.ibm.com>
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>