Rename HOSTNAME variable to TARGET_HOSTNAME

Bash sets the HOSTNAME variable, overriding our default

build.sh

@@ -162,7 +162,7 @@ export DEPLOY_DIR=${DEPLOY_DIR:-"${BASE_DIR}/deploy"}
 export DEPLOY_ZIP="${DEPLOY_ZIP:-1}"
 export LOG_FILE="${WORK_DIR}/build.log"
 
-export HOSTNAME=${HOSTNAME:-raspberrypi}
+export TARGET_HOSTNAME=${TARGET_HOSTNAME:-raspberrypi}
 
 export FIRST_USER_NAME=${FIRST_USER_NAME:-pi}
 export FIRST_USER_PASS=${FIRST_USER_PASS:-raspberry}