Zynq: Enable booting from unified image

This change uses the multiboot address register
in the SPL to read fields from the BootROM header
in SPI flash and subsequently calculate an offset to 
the u-boot image which is appended to the SPL image.
This means we can have one image containing SPL and
u-boot instead of two.

I do not expect this commit to be merged as is, but maybe it can serve as a starting point for a discussion
on how this could best be implemented.