Add a new "checksum" make target which generates an sha256sums file over the
image files produced in bin/targets/ and automatically call it during make
world after the package index generation.

The advantage of this new target is that it is guaranteed to run after the
images, the SDK and the ImageBuilder archives have been generated to ensure
that they all end up in the checksum file. Fixes FS#51.

Uses sed to postprocess the OpenSSL digest output into an sha256sum command
compatible format.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>