feat: added mainsailos module (#81)

* feat: added mainsailos module

This module provides a mainsailos-release file according to:
mainsail-crew/mainsail@cb87b4c

Signed-off-by: Stephan Wendel <me@stephanwe.de>

* style: added trailing new line

Signed-off-by: Stephan Wendel <me@stephanwe.de>

src/modules/mainsailos/config

@@ -0,0 +1,2 @@
+# This file is intentionally left blank
+# Preparation for later use cases

src/modules/mainsailos/start_chroot_script

@@ -0,0 +1,23 @@
+#!/usr/bin/env bash
+# MainsailOS Specific Tweaks
+# written by Stephan Wendel aka KwadFan
+# <me@stephanwe.de>
+# GPL V3
+########
+
+
+# Source error handling, leave this in place
+set -xe
+
+# Source CustomPIOS common.sh
+source /common.sh
+install_cleanup_trap
+
+# Create mainsailos release file
+if [ -f "/etc/mainsailos_version" ]; then
+    sudo rm -f /etc/mainsailos_version
+fi
+function get_parent {
+cat /etc/os-release | grep VERSION_CODENAME | cut -d '=' -f2
+}
+echo "${DIST_NAME} release ${DIST_VERSION} ($(get_parent))" > /etc/${DIST_NAME,,}-release