[Task]: Deprecate `rk3588-legacy` in favor of `rk3588-vendor`

[ColorfulRhino]

With a soon-to-be-updated linux-rockchip 6.1-rkr3 kernel, it may be time to finally sunset the 5.10 legacy kernel for RK3588.
The 6.1 vendor kernel is overall just a better/more up-to-date version of the Rockchip kernel and will receive updates down the line. With the recent addition of the current kernel branch for RK3588, we currently have 5 active kernel branches for this SOC: https://github.com/armbian/build/blob/main/config/sources/families/rockchip-rk3588.conf
It's also easier for @amazingfate to maintain only one branch of the kernel.

This task/issue is a place to discuss this topic before implementation. Let's hear your opinions 😄

Task List

For marking the legacy kernel as deprecated/EOL (now):

• rk35xx: Remove legacy kernel target from board configs #6881
• Change KERNELPATCHDIR to default for vendor in the family config
• Make 6.1 the default branch in https://github.com/armbian/linux-rockchip @amazingfate

For removing the legacy kernel as a whole (TO-DO at a later date):

• Make legacy and vendor point to the same variables in the family config
• Remove kernel patch folder for rk3588-legacy
• Remove kernel config file
• Remove traces of the legacy rk3588 kernel in the code

[github-actions]

Jira ticket: AR-2394

[rpardini]

I've one use case for the legacy / 5.10 kernel: being able to write MAC addresses to the Realtek 2.5gbit (r8125/r8169) ethernets. See instructions at Mario's -- the program only builds and works on 5.10, not 6.1.

Otherwise, I use legacy on "production" systems -- if it is removed from Armbian, I'll be forced to userpatch the hell out of it.

[igorpecovnik]

We can remove it from board configs - "legacy" is anyway EOL label. And later remove source defs.

[rpardini]

Yes. I'd be happy if it's removed from KERNEL_TARGETS, since we can actually build BRANCH=legacy even if it isn't listed there.

But I'd keep the family code handling it, which amounts to a 10-line block (repeated twice, just like vendor, due to the rk35xx/rockchip-rk3588(-which-is-actually-rk35xx-LINUXFAMILY) duplication crazy.

[ColorfulRhino]

Okay we can do it like that if you prefer 👍
I'll keep this task open as a reminder to finally delete legacy at some point later down the line (in some months/a year? we can see and re-evaluate in a few months).

I have updated the task list to reflect this approach. Feel free to edit it :)