env: ubi: KConfig: add CONFIG_ENV_UBI_VOLUME_REDUND

Introduce the KConfig option CONFIG_ENV_UBI_VOLUME_REDUND for defining
the name of the UBI volume used to store the redundant environment.

Signed-off-by: Markus Klotzbuecher <markus.klotzbuecher@kistler.com>
Reviewed-by: Heiko Schocher <hs@denx.de>
Cc: Kyungmin Park <kmpark@infradead.org>

hs: get rid of stm32mp1* build errors