If a distfile changed its checksum (SHA256) without at the same time
also changing its version, the distfile cache needs to be purged.
This is done by verifying only for already existing distfiles that the
specified checksum matches. If it does not, the file and its entry in
/host/sourcers/by_sha256 are purged using the inode number of the file.
The second loop then as usual fetches the distfiles and verifies their
checksum against the list entry.
+ drbd-utils fix checksum and also build + cross build
d8eca52226