src/manifest: differentiate between missing and zero size
src/manifest: differentiate between missing and zero size

Potentially, we could have images that have a size of zero.
Despite they will be inappropriate in most cases, we should
differentiate a missing and a zero size by using '-1' for 'missing size'
and '0' for 'zero size'.

Signed-off-by: Enrico Joerns <a2e5f936c390f36494e46e71bcfbe365bb1e8c49@pengutronix.de>

jluebbe/rauc,jluebbe/rauc,ejoerns/rauc,rauc/rauc,rauc/rauc,ejoerns/rauc,rauc/rauc,ejoerns/rauc,jluebbe/rauc