lib: updatehub: Fix download block error

The current version aborts update when found last transfer block. Now,
system checks only at end of coap block transfer total size and install
if download is ok.

Signed-off-by: Gerson Fernando Budke <gerson.budke@ossystems.com.br>
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
(cherry picked from commit 1128eab3)