[mk] Clean the inter-dependencies of the checksum-vars.mk and checksum.mk files
Like for other *-vars.mk files, checksum-vars.mk should be includable without triggering the {bootstrap-,}dependencies associated to the actual checksum targets. This commit leaves only the variable definitions in checksum-vars.mk, moving the rest to checksum.mk This way, the checkout target does not artificially inherit from the 'digest' dependency which is useless when working with a checked out package.