fw_cfg: add API for file transfer.

fw_cfg: add API for file transfer.

This patch adds a file transfer interface to fw_cfg.  Intended to be
used for passing non-pci option roms and vgabios to seabios.  Namespace
is modeled after the existing cbfs filesystem support in seabios.

Reading the new FW_CFG_FILE_DIR entry returns a file list.
Fields there are in network byte order (aka bigendian).

aliguori: fix fw_cfg.h for multiboot.bin, add proper fw_cfg.h declarations,
          quiet fprintf() in fw_cfg.c

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
(cherry picked from commit abe147e0ce41c07e789f6627bcb01c905d1f9bbf)