Add gbp.conf

This makes releasing simpler
2 jobs for pureos/3.99pureos+libre5.2 in 1 minute and 52 seconds (queued for 4 seconds)
latest
Status Job ID Name Coverage
  Test
passed #170151
librem5
builtin_tests

00:00:54

 
  Pkg
passed #170152
librem5
build-debian-package

00:00:58