board/firefly_rk3288: Update ARM variant and fix kernel repo

This commit updates the two firefly_rk3288 defconfigs recently
added. First we switch to the ARM cortex-A17 core. Second,
we switch the kernel github access from SSH to HTTPS, which
is suitable for anonymous access.

Signed-off-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>