Eclipse Kura 3.1.1

This is a service release of Eclipse Kura. The primary purpose of this release is to address the following items:

• Fix build issues introduced with the exists-maven-plugin: https://github.com/eclipse/kura/issues/1747
• Fix issue with unecessary delays when configuring firewall: https://github.com/eclipse/kura/issues/1761
• Fix GSM modem detection bug: https://github.com/eclipse/kura/issues/1766
• Minor fixes for typos in BLE services and documentation
• Minor fix for success notification with Eclipse Marketplace