image_types_phosphor: Rename mk_nor_image()

The mk_nor_image() function just creates an empty image file. Rename it
to mk_empty_image() so that it makes more sense when used to create a
file for other chips such as a mmc instead of a nor, like an empty rwfs
ext4 image file.

(From meta-phosphor rev: fdb81a30d7e86309aeafc6b6ab1ebf8f9cf93d88)

Change-Id: I31231ddaec5a8c195da315841e59ecdfb16647a5
Signed-off-by: Adriana Kobylak <anoo@us.ibm.com>
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>