sandbox: Filter arguments when starting U-Boot

The current method of starting U-Boot from U-Boot adds arguments to pass
the memory file through, so that memory is preserved. This is fine for a
single call, but if we call from TPL -> SPL -> U-Boot the arguments build
up and we have several memory files in the argument list.

Adjust the implementation to filter out arguments that we want to replace
with new ones. Also print a useful error if the exec() call fails.

Signed-off-by: Simon Glass <sjg@chromium.org>