script/seccomp.sh: check tarball sha256
script/seccomp.sh: check tarball sha256

Add checking of downloaded tarball checksum.

In case it doesn't match the hardcoded value, the error is like this:

	libseccomp-2.5.4.tar.gz: FAILED
	sha256sum: WARNING: 1 computed checksum did NOT match

In case the checksum for a particular version is not specified in the
script, the error will look like this:

	./script/seccomp.sh: line 29: SECCOMP_SHA256[${ver}]: unbound variable

In case the the hardcoded value in the file is of wrong format/length,
we'll get:

	sha256sum: 'standard input': no properly formatted SHA256 checksum lines found

In any of these cases, the script aborts (due to set -e).

Signed-off-by: Kir Kolyshkin <3a017b8ddb3f9cf3e4a59978b004111bdeb97f08@gmail.com>

opencontainers/runc,opencontainers/runc,opencontainers/runc,opencontainers/runc