fetch2: Revalidate checksums, YOCTO #5571

fetch2: Revalidate checksums, YOCTO #5571

[YOCTO #5571] -- https://bugzilla.yoctoproject.org/show_bug.cgi?id=5571

The following workflow (whether accidentally or deliberately) would
previously not result in a checksum error, but would be helpful to do
so:
 - Write a recipe with correct checksums
 - Fetch the sources for this recipe using bitbake
 - Change the checksums
Since the bitbake fetcher writes a done stamp after the initial download
and does not verify the checksums again (even if they are changed in the
recipe afterwards), the change of checksums is silently ignored.

Fix this without the overhead of computing the checksums from scratch on
every do_fetch by storing them in pickled format in the done stamp and
verifying that they still match those in the recipe.

Signed-off-by: Clemens Lang <clemens.lang@bmw-carit.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>