u-boot-at91: use environment size from machine config
u-boot-at91: use environment size from machine config

Use the correct size, according to boot medium.

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

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