APT: Add source.list and key for FFHO repo for Debian Buster.

Signed-off-by: Maximilian Wilhelm <max@sdn.clinic>

apt/ffho.list.Debian.buster

@@ -0,0 +1,2 @@
+deb	http://apt.ffho.net buster contrib non-free main
+deb-src	http://apt.ffho.net buster contrib non-free main