rk3588: Enable current kernel branch for some boards

[ColorfulRhino]

Description

Enable the current kernel branch for some RK3588 boards, which was introduced in #6699

current is kernel version 6.8 at the moment and should stay a more stable alternative than edge, which is often a rc kernel. This is useful when using the board as daily driver for example or when you don't want anythign at all to suddenly break.
current will stay at the next LTS kernel once released.

I did not enable it for every board at once, only the NanoPC T6 and the CM3588 NAS for now.
Feel free to add a commit to this PR or let me know if you'd like to enable current on a specific board.

How Has This Been Tested?

• ./compile.sh BOARD=nanopc-cm3588-nas BRANCH=current RELEASE=trixie EXPERT=yes KERNEL_CONFIGURE=no BUILD_MINIMAL=no BUILD_DESKTOP=no

Checklist:

• My code follows the style guidelines of this project
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules