auc: several improvements

 * include version_code in request
 * include versions of selected packages in request
 * add SHA256 verification via busybox sha256sum
 * sort attributes in policies alphabetically
 * move all API-specific string constants to precompiler macros
 * set correct MIME type for JSON post request (application/json)
 * output string error message if something goes wrong
 * auto-generate version string

Signed-off-by: Daniel Golle <daniel@makrotopia.org>