u-boot: use default environment if booting from NOR

[Werner Almesberger]

Patch nor-default-env.patch (revision 3973) makes u-boot on GTA02
always use the built-in default environment if booting from NOR.
That way, u-boot cannot be corrupted by whatever evils may lurk
in the environment stored in NAND.

Not in series yet. Will get there soon.

- Werner

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

nor-default-env.patch:
- cpu/arm920t/start.S, common/env_common.c (env_relocate): new configuration
  option CFG_DEFAULT_ENV_IF_NOR to use the default environment (and not try to
  load it from NAND) if booting from NOR
- cpu/arm920t/start.S (booted_from_nor): set this flag if booting from NOR
- include/configs/neo1973_gta02.h: set CFG_DEFAULT_ENV_IF_NOR