contrib: add SHA512SUM check test to restbed

Target .sum-restbed is missing from rules.mak and causes the execution error
of command "make fetch-all" in contrib.
Added the SHA512SUM file with sha512sum result on restbed archive to
call the default check SHA512 target.

Change-Id: Ic5b6b6b5bcf827cceb6999046cc9fa6917068c1e
Reviewed-by: Stepan Salenikovich <stepan.salenikovich@savoirfairelinux.com>