pkg-infra: add possiblity to check downloaded files against known hashes

Some of the packages that Buildroot might build are sensitive packages,
related to security: openssl, dropbear, ca-certificates...

Some of those packages are downloaded over plain http, because there is
no way to get them over a secure channel, such as https.

In these dark times of pervasive surveillance, the potential for harm that
a tampered-with package could generate, we may want to check the integrity
of those sensitive packages.

So, each package may now provide a list of hashes for all files that needs
to be downloaded, and Buildroot will just fail if any downloaded file does
not match its known hash, in which case it is removed.

Hashes can be any of the md5, sha1 or sha2 variants, and will be checked
even if the file was pre-downloaded.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Baruch Siach <baruch@tkos.co.il>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Gustavo Zacarias <gustavo@zacarias.com.ar>
Reviewed-by: Samuel Martin <s.martin49@gmail.com>
Cc: Thomas De Schampheleire <patrickdepinguin@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>