vexpress: Add support for the -bios flag to provide firmware

vexpress: Add support for the -bios flag to provide firmware

Right now to run firmware inside the QEMU VExpress model requires
padding out the firmware image to the size of the virtual flash and
passing it in via the -pflash argument. If the firmware image is passed
without padding, then QEMU will fail. Also, when passed as a -pflash
argument, QEMU treats the file as persistent storage and will modify the
file.

The -bios flag provides the semantics that we want for providing a
firmware image. This patch maps the contents of the -bios file into the
address space at the boot flash location.

Tested with the vexpress-a15 model and the Tianocore port.

Signed-off-by: Grant Likely <grant.likely@linaro.org>
Tested-by: Roy Franz <roy.franz@linaro.org>
[PMM: folded long line, removed stray \n from error message,
 use correct variable for printing image name, exit(1) rather than 0]
Reviewed-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>