ARM64: zynqmp: Add support for standard distro boot commands

Nand and QSPI are not defined now but this will be extended.
Based on selected bootmode boot_targets are rewritten.
Patch also contains detection if variables are saved. If yes don't
rewrite boot_targets variable.

Also move variable setup to the end of file because SCSI needs to be
defined before others macros are using it.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Alexander Graf <agraf@suse.de>