build-image: Skip message if board package list is empty

Otherwise it can be confusing.

Broken-by: c7f52db8

build-image

@@ -124,7 +124,7 @@ bootimg="${pureos_board}.img"
 [ -n "${debootstrap_dist}" ] || debootstrap_dist="${distro}"
 echo "Building ${bootimg} (board_type: ${board_type}, arch: ${arch}, build_type: ${build_type}, ci_pkgs: ${ci_pkgs}, ${debootstrap_dist})"
 echo "Installing extra packages ${packages}"
-echo "Installing extra board specific packages ${board_packages}"
+[ -z "${board_packages}" ] || echo "Installing extra board specific packages ${board_packages}"
 
 mkdir -p tmp/ files/
 echo -n > files/meta.yml