As far as I know phh released the kernel in his github, because in fact he used some of the things of my kernel from kitkat to lcd timings for example. As far as I know, rockchip changed his method in kernel 4.4xx to overclock it, it's not as easy as in kitkat kernel that only required to remove a security flag. I didn't look too much into new kernel because I don't have an Xd anymore, but I didn't find a way to unlock to 1,8ghz, just to 1,6 ghz (including 1,8 speeds in dts didn't work, so I guess there is a new flag somewhere in kernel avoiding that.
Anyhow, only PHH knows which SDK he used. Maybe can be used some firefly RK3399 SDK which is avaialble I think, but it would requite to make a new device tree and some work. Not an easy task. I know from experience because my legacyrom I used firefly kitkat SDK and I was forced to use some stuff of stock roms because I didn't have access to GPD SDK. I also asked GPD several times if they had some rockchip SDK for later versiones without success. Probably because they only wanted to sell their new XD+ device. (that device had a full SDK release in a a private way that I leaked it to Black-Seraph) and it provoked GPD fury against me.
I also never checked the boot.img that phh used. Rockchips boot.img can be kernel binary+ramdisk or just a ramdisk, so if it's the first case you need to extract boot.img to include your kernel. If it's just a ramdisk then kernel.img is what is used for kernel. Also, nougat rockchip uses some partitions that are not used in Kitkat/lollipop roms. To be honest, the best way would be that phh leaked privately to you the SDK he used, to have a good starting point to begin to work if you are interested.