config: remove distro_default from common.h

Including it from here doesn't ensure that the config options show up
in all the right places. Instead, each per-board header file (or a file
that boards include) should include it themselves directly.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
---
This series is based on:
a) Dennis Gilmore's series to add config_distro_default.h. Since that
   series is RFC, so is this.
b) My recent series to implement the "exists" shell command.
ARM: tegra: convert tegra to use distro defaults

Modify all Tegra boards to include the "distro defaults" header, so that
all the config options distros expect are enabled. Remove any #defines
that enable the same options from the Tegra files.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
ARM: tegra: rework boot scripts

Update the common Tegra boot scripts in the default environment to

a) Make use of the new "exists" shell command to avoid some error
   messges.

b) Allow booting using the sysboot command and extlinux.conf. This
   allows easy creation of boot menus, and provides a simple interface
   for distros to parameterize/configure the boot process.

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