ARM: zynq: Add target for building bootable SPL image for Zynq

Add a build target to generate 'boot.bin' which includes SPL. This is
used by the platforms BootROM to load SPL directly.

This change also conditionally changes what the 'boot.bin' target
generates depending on the SoC. Leaving the behaviour unchanged for the
AT91 targets.

Signed-off-by: Nathan Rossi <nathan@nathanrossi.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Cc: Tom Rini <trini@konsulko.com>
Cc: Andreas Bießmann <andreas.devel@googlemail.com>
Cc: Heiko Schocher <hs@denx.de>
Reviewed-by: Tom Rini <trini@konsulko.com>