Kconfig support in build2

[Boris Kolpackov]

Just a short note if anyone is looking for a build system with an
out-of-the-box Kconfig support:

https://build2.org/blog/build2-kconfig.xhtml


As part of this effort we've also written a Kconfig language guide,
finding the official documentation to be more of a reference:

https://build2.org/libbuild2-kconfig/doc/build2-kconfig-manual.xhtml#lang