devel/release: also generate the archive's SHA256
devel/release: also generate the archive's SHA256

MD5 is broken, SHA1 is being phased out, SHA256 should be used wherever
possible.

While at it, use a variable for the archive name, which will make
changing the archive format (e.g. to use xz) easier in the future.

Signed-off-by: Apollon Oikonomopoulos <ed81ed0a45ccd5c8869bda52e38353a6299ee39e@dmesg.gr>

ganeti/ganeti,mbakke/ganeti,ganeti/ganeti,ganeti/ganeti,mbakke/ganeti,mbakke/ganeti