Dynamic source include for kconfig

[Luis Chamberlain]

Hey Masahiro,

I ended up needing to do a dyanmic include of kconfig files to support
custom / private vagrant boxes in a project I maintain.  I figured out
a way to do it, and not sure if it was intended, but it works:

https://github.com/mcgrof/kdevops/commit/404a55326198787758fac88af3c60dcaecf599fc

A new helper for this sort of thing might be neat, if one follows a
convention. Thoughts?

 Luis