debian: Add md5 sum fallback when sha* checksum is missing in metadata

In order to check successful download of a package file, the debian loader
will compare sha256 or sha1 checksum of the file with the one located in
debian dsc file.

However for old debian-based distributions (some ubuntu old releases for
instance) the only available checksum in the dsc file is a md5 sum.

So add a fallback to use md5 sum to check successful download when sha*
checksum is missing in the dsc file.

Related to T2400