Use existing common configuration to setup PREFIX

Signed-off-by: Martyn Welch <martyn.welch@collabora.co.uk>

dbus

 #!/bin/sh
 # vim: set sts=4 sw=4 et tw=0 :
 
-case `uname -m` in
-x86_64) PREFIX=amd64/bin; ;;
-armv7l) PREFIX=armhf/bin; ;;
-aarch64) PREFIX=arm64/bin; ;;
-esac
-
 echo "$0: setup"
+. common/update-test-path
 . common/common.sh
 ensure_dbus_session