arm_fpga: Support uploading a custom command line

The command line for BL33 payloads is typically taken from the DTB. On
"normal" systems the bootloader will put the right version in there, but
we typically don't use one on the FPGAs.
To avoid editing (and possibly re-packaging) the DTB for every change in
the command line, try to read it from some "magic" memory location
instead. It can be easily placed there by the tool that uploads the
other payloads to the FPGA's memory. BL31 will then replace the existing
command line in the DTB with that new string.

To avoid reading garbage, check the memory location for containing a
magic value. This is conveniently chosen to be a simple ASCII string, so
it can just preceed the actual command line in a text file:
--------------------------------
CMD:console=ttyAMA0,38400n8 debug loglevel=8
--------------------------------

Change-Id: I5923a80332c9fac3b4afd1a6aaa321233d0f60da
Signed-off-by: Andre Przywara <andre.przywara@arm.com>