raspberrypi-ua-netinst v2.4.0

Changelog:

• support for version 4 model B
• support for Debian 10 (buster)
• removed handling of Debian 7 (wheezy) because it is no longer available in Raspbian repository
• added or changed configuration variables:
  • release is set to "buster" by default

Bugfixes:

• upgrade initial packages