build-update-package: Added check to verify the existence of .md5sum dir

Exit on error 5, in case there is no .md5sum directory.

Signed-off-by: Erik Alexandre Pucci <eap08@c3sl.ufpr.br>
Acked-by: Adriano da Luz <al08@c3sl.ufpr.br>
Acked-by: Josiney de Souza <josineys@c3sl.ufpr.br>