u-boot: environment override

[Werner Almesberger]

Revision 4035 adds a new feature and removes an old one: we can now
override the environment by putting a pointer to an environment block
at offset 0x44 (or 0x040, if preboot-override is disabled) of the
u-boot binary.

The devirginator uses preboot-override, and for NOR we know use that,
plus the new environment override (trunk/src/host/devirginator/mknor).

In our scenario this means that we don't need nor-default-env.patch
anymore, which I've therefore removed from "series". (The
nor-default-env.patch file is still around, in case we still find some
use for it.)

- Werner

----------------------------------- Changes -----------------------------------

env-override.patch:
- cpu/arm920t/start.S: new configuration variable CFG_ENV_OVERRIDE to enable
  environment overriding
- cpu/arm920t/start.S: added global variable "env_override" to specifiy an
  alternate initial environment
- common/env_common.c (default_env): if env_override is set, use that
  environment as the default

uboot-gta02.patch:
- include/configs/neo1973_gta02.h: set CFG_ENV_OVERRIDE

series:
- added env-override.patch
- backed out nor-default-env.patch, since env-override.patch provides the same
  functionality (in our context)