Build & install dlt-viewer(v2.21.2) on Ubuntu 20.04

1. Cloning the dlt-viewer from Github. (https://github.com/GENIVI/dlt-viewer)
2. Checking out with proper tag. (it’s v2.21.2 now)
3. Executing doc/convert.sh to generate pdf docs which need to build. (You may need to install ‘asciidoc-base’ package)
4. Executing build_viewer_debs.sh. (You may need to install several packages such as devscripts, qt5-default, debhelper, libqt5serialport5-dev and so on)
5. You can find deb packages in debtmp/.