cea/build_preseed.sh: Allow trigger script from top-level

Refs. sysadm-environment#5367

cea/build_preseed.sh

@@ -9,7 +9,7 @@ CONFIG="${BASEDIR}/configs/${HOSTNAME}.yaml"
 ROOT_PASSWORDS_FILE="${BASEDIR}/root_passwords"
 SWH_PASSWORDS_FILE="${BASEDIR}/swh_passwords"
 
-mkdir -p configs/preseeding/finish_install
+mkdir -p ${BASEDIR}/configs/preseeding/finish_install
 
 generate_and_save_password() {
     local PASSWORDS_FILE=$1