For the Arm Ltd. FPGAs to run, we need to load several payloads into the
FPGA's memory:
- Some trampoline code at address 0x0, to jump to BL31's entry point.
- The actual BL31 binary at the beginning of DRAM.
- The (generic) DTB image to describe the hardware.
- The actual non-secure payloads (kernel, ramdisks, ...)

The latter is application specific, but the first three blobs are rather
generic.
Since the uploader tool supports ELF binaries, it seems helpful to
combine these three images into one .axf file, as this also simplifies
the command line.

Add a post-build linker script, that combines those three bits into one
ELF file, together with their specific load addresses.
Include a call to "ld" with this linker script in the platform Makefile,
so it will be build automatically. The result will be called "bl31.axf".

Change-Id: I4a90da16fa1e0e83b51d19e5b1daf61f5a0bbfca
Signed-off-by: Andre Przywara <andre.przywara@arm.com>