buildroot

boot/at91bootstrap3: download via custom git url

This patch adds the option to download at91bootstrap3 from
a custom git URL. This is a requirement for all that boards
that use a custom at91bootstrap3 version.

Signed-off-by: Angelo Compagnucci <angelo.compagnucci@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>