env: Add option to only ever append environment

Add configuration option which prevents the environment hash table to be
ever cleared and reloaded with different content. This is useful in case
the first environment loaded into the hash table contains e.g. sensitive
content which must not be dropped or reloaded.

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Tom Rini <trini@konsulko.com>