arm: zynq: Move bootmode to headers

These numbers can be reused.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
zynq: Use board_eth_init if CMD_NET is not enabled

board_eth_init can be also called in cases where CMD_NET
is not enabled.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>