image_types_phosphor: Split make_rwfs()

The make_rwfs() was doing some cleanup before creating the rwfs file.
Split it into two functions clean_rwfs() and make_rwfs() for cases where
some prep work is needed before calling the rwfs command, such as when
using mkfs.ext4 that needs to have a file created before it's converted
into ext4.

(From meta-phosphor rev: e82e8bc686b0a17f29d1d296f1d52543f2bc5ae0)

Change-Id: Idebaab86bfd6ff0c02444d9c62543e97e9ba0dcd
Signed-off-by: Adriana Kobylak <anoo@us.ibm.com>
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>