hash.sh returns just the sha256sum of its input file
checkhash.sh uses ha.sh to get the sha256sum of its first argument and
then validates that the checksum matches the value provided in its
second argument
hash.sh isn't currently fully portable since sha256sum isn't present
by default on macOS, but it provides a single point around which we can
do hashing to validate checksums
Signed-off-by: Ashley Davis <ashley.davis@jetstack.io>
