env: split env_import_redund() into 2 functions

split from env_import_redund() the part which checks
which Environment is valid into a separate function
called env_check_redund() and call it from env_import_redund().

So env_check_redund() can be used from places which also
need to do this checks.

Signed-off-by: Heiko Schocher <hs@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>