Buck supports building Java-based Android projects. There is a project https://github.com/zserge/buckbone that adds "experimental" Kotlin support but it's far from being usable.
Is there a way to build an Android app written in Kotlin using Buck build system?
There isn't currently a great way to do this, but zserge has been the one dabbling in it (check out some of the issues they've filed in Buck). scrawlings actually recently did a refactor of Buck's Java code to make support for Groovy easier, which would make it a lot easier to fully support Kotlin too. The core team isn't likely to work on that, but we'd certainly take a PR for it.