ARM: zynq: Run distribution boot commands first

This patch is doing two things.
1. Exchanging order of boot commands. distro_bootcmd is run first
followed by Xilinx boot command.
2. Remove CONFIG_BOOTCOMMAND from configs (and follow mainline) by
creating Xilinx distribution bootcommand and wiring it as the last
bootcommand.

QSPI, NAND distribution boot command will be added later.

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