mvebu: omnia: make initramfs image usable out of the box

mvebu: omnia: make initramfs image usable out of the box

Currently it's not possible to boot the device with just initramfs image
without additional effort as the initramfs image doesn't contain device
tree.  Fix it by producing FIT based image which could be booted with
following commands:

 setenv bootargs earlyprintk console=ttyS0,115200
 tftpboot ${kernel_addr_r} openwrt-mvebu-cortexa9-cznic_turris-omnia-initramfs-kernel.bin
 bootm ${kernel_addr_r}

Acked-by: Klaus Kudielka <klaus.kudielka@gmail.com>
Reviewed-by: Tomasz Maciej Nowak <tmn505@gmail.com>
Signed-off-by: Petr Štetiar <ynezz@true.cz>
(cherry-picked from commit 337ff74894110b35b61118918b7eb30bb6e60756)