Remove code duplication for setting the default environment

Remove code duplication for setting the default environment

common/env_common.c (default_env): new function that resets the environment to
		    the default value
common/env_common.c (env_relocate): use default_env instead of own copy
common/env_nand.c (env_relocate_spec): use default_env instead of own copy
include/environment.h: added default_env prototype

Signed-off-by: Werner Almesberger <werner@openmoko.org>
Signed-off-by: Harald Welte <laforge@openmoko.org>