This patch splits the download of signatures from the download of the
archive to improve robustness. This way, signature files will be
downloaded even if the corresponding archive is already in place.
Issue #748
This patch makes the handling of failed integrity checks of 3rd-party
packages more robust. Previously, a once failed 'make prepare PKG=curl'
would not leave any trace of the verification state. Hence, a successive
attempt to perform the 'make prepare' step again succeeded even if the
signature check failed.
To solve this problem, the outcome of a successful signature check is
represented by a tag file called 'download/<archive-name>.verified'.
Because the rule for extracting the archive depends on the .verify tag
file, the extraction step is not performed until the signature check
succeeds.
Issue #748
