baseimager.py: rename MANIFEST to MD5SUMS

baseimager.py: rename MANIFEST to MD5SUMS

The name "MANIFEST" is not very good for multiple reasons:

1. It is not user-friendly - mere users do cannot tell what this file is from
   its name.
2. Tizen is currently the most important user of MIC, and in Tizen we use word
   "manifest" for a different thing - every package has a security manifest
   file which declares the needs of the package.
3. MIC stores checksums in the MANIFEST, and this file is compatible with the
   'md5sum' tool. If we decide to provide sha1 checksums in the future, we'll
   have to add another file with a different name. What will that name be?

Anyway, it is nicer and cleaner to use the same naming scheme as OpenSuse and
Ubuntu use. They have MD5SUMS and SHA1SUMS files.

Let's make things more aligned and cleaner and rename MANIFEST into MD5SUMS.

This may break some scripts some people use. But I think it is anyway a low
risk thing, and it is better to rename now, when Tizen has few users. It may be
more difficult to do later.

Change-Id: I79f968066aa27bd211622f477b27a1973b3775ad
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@intel.com>