Cross-compiling Beaglebone on amd64

I am not exactly sure but my searching took me to /usr/src/share/mk/
From there I did a search on and discovered relevant files in /usr/src/sys/conf/

I am short on time but I would gander that there is something relevant in this directory.

In this post some users show how to add modules to make.conf:
# Modules to build instead of all
# Modules not to build
# WITHOUT_MODULES = linux acpi

Usually anytime there is a WITHOUT_ option there is also a corresponding WITH_ option.
but what if I want to build a couple of RPi modules too?
Wouldn't you just add them to the kenconf? That is what I do for onewire.

As an alternative you might be able to compile them with -DWITH on the command prompt when running make.
I found the correct option, which can be used for a particular board.
In the configuration files (per board) in /usr/src/sys/arm/conf/:
makeoptions MODULES_EXTRA="module1 module2 module3"
However, supplying it directly on the makecommand line doesn't work.