uboot-airoha: fix eMMC boot failure caused by redundant env

uboot-airoha: fix eMMC boot failure caused by redundant env

The an7581/an7583 configs enable redundant environment support,
but do not define CONFIG_ENV_OFFSET_REDUND.

When running `saveenv`, U-Boot tries to write the redundant environment
to MMC. Without a valid redundant environment offset, the data may be
written to an incorrect location and overwrite critical boot data such
as the GPT, BL2/preloader or BL31+U-Boot FIP.

This causes the board to fail booting after saving the environment.

Define CONFIG_ENV_OFFSET_REDUND so the redundant environment is stored
at a valid eMMC offset and no longer corrupts the bootchain.

Signed-off-by: Yalei Zang <yalei.zang@airoha.com>
Link: https://github.com/openwrt/openwrt/pull/24397
Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>