You can build the kernel on the VIM or on a PC (using cross compilation).
This is the source code for the kernel used in the latest version MultiOS_3in1. In Armbian use other kernel sources.
Now there is not much confusion in the designation of the branches of u-boot. A branch in git Khadas MultiOS - does not support muliboot (not running multiple systems from removable media SD cards and USB flash drives). This version should be called DualOS (she supports the running of two systems of eMMC ). I use the branch Nougat-Ubuntu (in my git). In my branch there is support for running systems with external media and run different systems from eMMC. Pay attention to compile u-boot must be performed with the version of gcc 4.9.