Yes, most of this is all supported. There is a existing recipe for building an image that you just dd to and SD card and can boot in a Raspberry Pi (and other devices supported by mainline kernel + u-boot).
Kernel is a package just like another; so you just write your own recipe. Patching kernel recipe works just like patching any Nix recipe.
Not sure about the rockchip-proprietary boot parts; you may be on your own.
The huge boon of Yocto is that it solves the Rockchip-proprietary parts. And the Qualcomm-proprietary parts. And the IMX-proprietary parts. It has BSP layers which you can add, you can use either e.g Rockchip's official Yocto BSP which focuses on doing things their way + using a vendor kernel or you can use the Yocto project's community Rockchip BSP which focuses on doing things the Yocto way + using upstream.
Even devices which are supported by mainline Linux + uBoot have their own ways you have to do e.g partitioning or what needs to be in Flash. Lots of Rockchip devices are perfectly supported upstream, but their boot process expects to find a tiny loader at a particular flash offset which can then jump to uBoot. Nothing in the ARM SBC world is standardized so everyone does things like this (tho some have partitioning schemes you need to follow instead of using offsets). That's stuff you can't really ignore if you want to be relevant in this space.
Kernel is a package just like another; so you just write your own recipe. Patching kernel recipe works just like patching any Nix recipe.
Not sure about the rockchip-proprietary boot parts; you may be on your own.