build_image: refresh scripts bundled into the update zip.
build_image: generate update tools zip by default

Missed this in 7231b95a, the update zip should still be built when the
usr partition is extracted for generating updates but build_image itself
is not generating and signing the update.
updates: add example scripts for signing update payloads

This pretty much just translates the signing instructions into some
trivial wrapper scripts to serve as an example on how to do this.
Merge pull request #285 from marineam/version2

Refresh the update tools zip