scripts/get_default_envs.sh: preserve order of multiple entries for same variable

It's possible that the default_environment[] array contains multiple
entries for the same variable, e.g. a setting from env_default.h based
on some CONFIG_* variable, and another from
CONFIG_EXTRA_ENV_SETTINGS. In such a case, the last setting takes
effect.

Hence, in order to be able to use the output from this script as an
CONFIG_DEFAULT_ENV_FILE and get the same default environment as one
currently has, we need to preserve the order. So only sort by the
variable name, and disable the last-resort comparison.

We could pipe the result through uniq to remove duplicate lines, but I
think there's some value in seeing that certain variables are defined
multiple times.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Reviewed-by: Lukasz Majewski <lukma@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>