scripts: declare ROOT closer to where it's used, and some DRY changes

This also prevents shellcheck from complaining about a possibly
undefined variable.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

test/build.sh

@@ -22,7 +22,10 @@ set -o errexit
 set -o nounset
 set -o pipefail
 
-source "$(dirname "${BASH_SOURCE[0]}")/build-utils.sh"
+basedir="$(dirname "${BASH_SOURCE[0]}")"
+source "${basedir}/build-utils.sh"
+
+ROOT="$( cd "${basedir}" && pwd )"/..
 cd "${ROOT}"
 
 # Make sure output directory is clean.