board: MCR3000: Modernise the settings to properly work on lastest u-boot version

Both U-boot and Linux kernel have grown over the last releases
and don't fit anymore in the 2M EPROM of the board.

So, rework the setup to allow storing the Linux kernel image
on the UBIFS NAND Flash.

Also add support to FIT images as this is what the Linux kernel
look like nowadays.

Also increase CFG_SYS_BOOTMAPSZ to 32Mbytes and define
CONFIG_SYS_BOOTM_LEN with the same value, otherwise it defaults
to 8M which is not sufficient anymore with nowadays Linux kernels.

And set the netmask to 255.255.255.0 as a class C address is used.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Reviewed-by: FRANJOU Stephane <stephane.franjou@csgroup.eu>