u-boot: always enter the boot menu if booting from NOR (GTA02)

[Werner Almesberger]

As previously discussed, I've now made u-boot always enter the boot
menu when booting from NOR.

Interestingly, this helps to implement a solution for the crash on
interrupt issue in u-boot that was recently reported. More about this
in a moment.

- Werner

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

This patch makes u-boot on GTA02 always enter the bootmenu when booting from
NOR. This de-complexifies interaction with the user and is actually also 
needed for correctly handling the exception vectors.

nor-bootmenu.patch:
- cpu/arm920t/start.S (booted_from_nor): set this flag if booting from NOR
- board/neo1973/gta02/gta02.c (board_late_init): always enter the boot menu if
  booting from NOR
- board/neo1973/gta02/gta02.c (board_late_init): used the opportunity to place
  the "extern"s at a more prominent location