[KERNEL][U-BOOT] Use branches based on MediaTek's release v24.1

Rebased the branches to the new state. Also included a new minor feature from MediaTek - export of the fw_env size for the flash tool.

https://git.seco.com/clea-os/bsp/mediatek/linux-seco-mtk/-/commits/seco_v5.15_v24.1
https://git.seco.com/clea-os/bsp/mediatek/u-boot-seco-mtk/-/tree/seco_v2022.10_v24.1

Note
Default branches in Kernel/u-boot repos need to be updated after merging!