Helps to avoid cases with broken rpi3 firmware archive, which happens
when build process was interrupted or because of network issues.

Reviewed-by: Joakim Bech <joakim.bech@linaro.org>
Reported-by: Bjorn Wennberg <bjorn.wennberg@forgerock.com>
Signed-off-by: Igor Opaniuk <igor.opaniuk@linaro.org>