buildroot

arm-trusted-firmware: exclude from hash when downloading from git

Since we will enable hash checks for git downloads soon, the hash check
for the custom git download should be disabled.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>