mvebu: image: add check for fdt_add_r and kernel_addr_r variables

mvebu: image: add check for fdt_add_r and kernel_addr_r variables

fdt_addr and kernel_addr variables are getting obsolete in the mainline
u-boot in favor of fdt_addr_r and kernel_addr_r.

By checking if the new variables exist, we can make sure that devices with newer
version of u-boot will work while not breaking support for the existing ones.

Signed-off-by: Vladimir Vid <vladimir.vid@sartura.hr>
Acked-by: Tomasz Maciej Nowak <tomek_n@o2.pl>