image_types_phosphor: Remove mtd from make_tar_of_images()

The make_tar_of_images() function was adding an mtd suffix to the
tarball name. Make the caller pass the suffix instead so that this
function can be used to create tarballs intended for non-mtd systems
such as systems based on a mmc chip.

(From meta-phosphor rev: 7bdbb7a6adaa655ba56ae2e289a59e13c9428b14)

Change-Id: I80ee15337f99ec59cce119668ef7f5afb2313189
Signed-off-by: Adriana Kobylak <anoo@us.ibm.com>
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>