env: ubi: add support of command env erase

Add support of opts erase for ubi env backend, this opts is used by
command 'env erase'.

This command only zero-fill the env UBI volume CONFIG_ENV_UBI_VOLUME
and CONFIG_ENV_UBI_VOLUME_REDUND, so the saved environment becomes
invalid.

This patch introduces a local define ENV_UBI_VOLUME_REDUND
only to avoid #if in the code, as CONFIG_ENV_UBI_VOLUME_REDUND
is only defined when CONFIG_SYS_REDUNDAND_ENVIRONMENT is defined.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>
Reviewed-by: Heiko Schocher <hs@denx.de>