pxe: implement fdtdir extlinux.conf tag

pxe: implement fdtdir extlinux.conf tag

People who write (or scripts that auto-generate) extlinux.conf don't
want to know about HW-specific information such as FDT filenames. Create
a new extlinux.conf tag "fdtdir" that specifies only the directory where
FDT files are located, and defer all knowledge of the filename to U-Boot.
The algorithm implemented is:

==========
if $fdt_addr_r is set:
  if "fdt" tag was specified in extlinux.conf:
    load the FDT from the filename in the tag
  else if "fdtdir" tag was specified in extlinux.conf:
    if "fdtfile" is set in the environment:
      load the FDT from filename in "$fdtfile"
    else:
      load the FDT from some automatically generated filename

if no FDT file was loaded, and $fdtaddr is set:
  # This indicates an FDT packaged with firmware
  use the FDT at $fdtaddr
==========

A small part of an example /boot/extlinux.conf might be:

==========
LABEL primary
        LINUX zImage
        FDTDIR ./

LABEL failsafe
        LINUX bkp/zImage
        FDTDIR bkp/
==========

... with /boot/tegra20-seaboard.dtb or /boot/bkp/tegra20-seaboard.dtb
being loaded by the sysboot/pxe code.

Signed-off-by: Stephen Warren <swarren@nvidia.com>