Create V2 signatures if the minimum target NC is 19

The v2 signature is also reproducable in other languages. As the hashes
are just stored as a single string that is signed. This is then also
verified.

To be backwards compatible such v2 signatures should only be generated
for apps that have a minimum version of NC19

Signed-off-by: Roeland Jago Douma <roeland@famdouma.nl>