unified list command for all scripts

e.g. ./build boot roms list
./update blobs inject listboards
./build boot list
./build clean list

also this is now possible:
./build list
or maybe
./update list
^ would list directories in resources/scripts/build
and resources/scripts/update respectively

this script is added:
resources/scripts/build/command/options

call it like so, e.g.
./build command options resources/coreboot

this script is now used, for list functions in
other scripts.

Signed-off-by: Leah Rowe <leah@libreboot.org>

fetch_trees

@@ -51,17 +51,11 @@ main()
 	[ -d "${cfgsdir}" ] || err "unsupported project name"
 	shift 1
 
-	targets=""
-	if [ $# -gt 0 ]; then
-		targets=$@
-	else
-		for x in "${cfgsdir}/"*; do
-			[ -d "${x}" ] || continue
-			targets="${targets} ${x##*/}"
-		done
-	fi
+	targets=$(./build command options "${cfgsdir}")
+	[ $# -gt 0 ] && targets=$@
 	[ -z "${targets}" ] && \
 	    err "No targets available for project: ${project}"
+
 	for x in ${targets}; do
 		rm -f "${cfgsdir}"/*/seen || err_rm_seen "main 2"
 		download_for_target "${x}" || \