dt-overlay-at91: make sure the kernel sources are available
dt-overlay-at91: make sure the kernel sources are available

DTBO compilation depends on having access to kernel source dir, so make
sure they are available.

Signed-off-by: Razvan Stefanescu <f4ee3fcb6e54dcf71fbaefdd500eea5cee1f4f99@microchip.com>

linux4sam/meta-atmel,alexandrebelloni/meta-atmel,alexandrebelloni/meta-atmel,linux4sam/meta-atmel,linux4sam/meta-atmel,alexandrebelloni/meta-atmel,linux4sam/meta-atmel,linux4sam/meta-atmel