Add OMV installation via armbian-config tool

Add build step for Armbian (replace linux page by armbian)

docs/armbian.md

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+# Armbian #
+
+Debian based Linux for ARM based single-board computers
+
+[https://www.armbian.com](https://www.armbian.com "Armbian")
+
+
+# How to build an image or a kernel?
+
+Supported build environment is **Ubuntu Bionic 18.04 x64** ([minimal iso image](http://archive.ubuntu.com/ubuntu/dists/bionic/main/installer-amd64/current/images/netboot/mini.iso)).
+
+- guest inside a [VirtualBox](https://www.virtualbox.org/wiki/Downloads) or other virtualization software,
+- guest managed by [Vagrant](https://www.vagrantup.com/). This uses Virtualbox (as above) but does so in an easily repeatable way. Please check the [Armbian with Vagrant README](https://docs.armbian.com/Developer-Guide_Using-Vagrant/) for a quick start HOWTO,
+- inside a [Docker](https://www.docker.com/), [systemd-nspawn](https://www.freedesktop.org/software/systemd/man/systemd-nspawn.html) or other container environment [(example)](https://github.com/armbian/build/pull/255#issuecomment-205045273),
+- running natively on a dedicated PC or a server (**not** recommended),
+- **20GB disk space** or more and **2GB RAM** or more available for the VM, container or native OS,
+- superuser rights (configured `sudo` or root access).
+
+**Execution**
+
+	apt-get -y install git
+	git clone https://github.com/armbian/build
+	cd build
+	./compile.sh
+
+Make sure that full path to the build script does not contain spaces.
+
+You will be prompted with a selection menu for a build option, a board name, a kernel branch and an OS release. Please check the documentation for [advanced options](https://docs.armbian.com/Developer-Guide_Build-Options/) and [additional customization](https://docs.armbian.com/Developer-Guide_User-Configurations/).
+
+Build process uses caching for the compilation and the debootstrap process, so consecutive runs with similar settings will be much faster.