builder.sh: use resilient curl calls to downloads.tf.org

From time to time, there has been issues, i.e. 'curl: (52) Empty reply
from server' [1], while fetching files from [2], so curl needs to be
more resilient on its execution.

[1] https://ci.trustedfirmware.org/job/tf-a-builder/66944/console
[2] https://downloads.trustedfirmware.org/

Signed-off-by: Leonardo Sandoval <leonardo.sandoval@linaro.org>
Change-Id: Ie0915c17613fc65e3beea01318ba14f0c477cf8f
1 file changed