[Phone] Building an Ubuntu Touch compatible kernel

I am currently trying to build an Ubuntu Touch-compatible keel for use with the Xperia Z5 Compact. I have been following the How to build and flash a Linux keel for AOSP supported devices guide and am at the point at which I have created an Ubuntu-compatible .config file but am having issues finally building the keel (Step 7) using the .config file as I receive the following error:

make ARCH=arm64 CROSS_COMPILE=$CROSS_COMPILE -j 14
Makefile:791: *** multiple target pattes. Stop.

I know that this error is related to vmlinux.sh which is referenced as follows on line 791 of the Makefile:

vmlinux: scripts/link-vmlinux.sh $(vmlinux-deps) FORCE

Any suggestions for debugging this problem would be greatly appreciated as googling has provided me no leads.