Hello. I am trying to do the EXACT same thing as OP. I want to compile OpenWrt with an MPTCP-enabled kernel. OpenWrt seems to have had an MPTCP project but its too old (last updated 5 years ago and using ancient kernel 3.14). For several days, I have tried many ways of doing this:
i) Use this guide of someone who accomplished the same thing for OpenWrt, but he is using a Raspberry Pi. I am stuck at "git fetching" from mptcp, because for some reason I don't know, the command starts fetching >6GB of data (which is weird because if I instead go to the repo and manually click download, I get a 170MB file. I don't know how to use git at all and I had to give up on this.
ii) Merge the source files manually, in a very archaic manner. Basically, I download MPTCP kernel (not to be confused with OpenWrt's MPTCP Kernel; this one is from the MPTCP developers) from here and then search for "mptcp" in the whole folder. I got about 50 files. Then I go to "/home/nero/openwrt/build_dir/target-mips_24kc_musl/linux-4.20" (this is where OpenWrt places the kernel source after downloading and extracting), then I replace the parts of code that are different from the mptcp kernel, one by one. Very time consuming and tiring. As you can imagine, it doesn't work when I try to compile. Mostly because its almost impossible to know what should be merged into what, without actually understanding the code. Which is thousands of lines. I gave up on this.
iii) I tried to make OpenWrt use their ancient kernel with MPTCP. It wont compile because of missing "gcc-compiler7.h". I tried all sorts of remedies to make that work, I eventually had to give up. See my post here
iv) Just delete the already downloaded kernel sources, and replace them with kernel sources of MPTCP (effectively tricking OpenWrt into compiling the kernel sources we want. Obviously, doesn't work (possibly because of Makefiles and Kconfigs). Gave up on this too.
v) Download an MPTCP kernel patch from here corresponding to the kernel system I am trying to patch. I am not sure how this works, but I think the patch is supposed to magically patch the whole kernel sources to support MPTCP (it is 22,000 lines, and from looking at the code, it seems to be doing what I was trying to do in (ii), albeit in a more sophisticated manner. It appears to me that just the patch file is enough to convert a raw, unmodified kernel source to support MPTCP. Again, not sure on this. After placing the patch in "target/linux/ath79/patches-420" and running make, it stops compiling at some point, due to the patch file. Some of the patch file does patch though, which makes me think this could be another solution. Again, no idea what I am doing here.
Right now what I think could work is to instruct OpenWrt to use a kernel from a different source. However, I don't know how to do that (It's possible though). If I can do that, then I can make it use the kernel from MPTCP and hopefully everything works.
I wish OpenWrt would work on this feature. I tested it before on my computer and I was able to merge WiFi and Ethernet into one super fast internet connection (the whole reason why I am trying to enable MPTCP on the router, so every device I have can benefit from this).