corstone1000: set kernel_addr based on boot_idx

corstone1000: set kernel_addr based on boot_idx

We need to distinguish between boot banks and from which
partition to load the kernel+initramfs to memory.

For that, fetch the boot index, fetch the correspondent
partition, calculate the correct kernel address and
then set the env variable kernel_addr with that value.

Signed-off-by: Rui Miguel Silva <rui.silva@linaro.org>