Add a script to generate sha256sums from Gitian output
Currently, the sha256sums files are assembled by hand (copy-pasting the hash results from the Gitian build).
This patch introduces make-sha256sums.sh to do this automatically.
make-sha256sums.sh -h make-sha256sums.sh # errors as expected make-sha256sums.sh [path to some gitian output] make-sha256sums.sh path/to/0.20.9 > jasonbcox-sha256sums.0.20.9
Reviewers: #bitcoin_abc, deadalnix, Fabien
Reviewed By: #bitcoin_abc, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D4893