Switch to newer U-Boot since it works reliably and provides upstream
improvements.
Add patch/u-boot/v2025.10 and the generic patch, as BOOTPATCHDIR cannot
be unset (fallback to legacy fails).
BOOT_SCENARIO="binman-atf-mainline" cannot be set here since it applies
globally to the CSC file and would break branch=vendor. Users who want
it can enable it manually.