tool/run: skip build if build-target list is empty

This patch prevents the run tool from issuing 'make' in the build
directory if no build targets are specified, which prompts the build
system to build everything.

tool/run/run

@@ -44,6 +44,8 @@ proc assert_spec {spec} {
 #
 proc build {targets} {
 
+	if {[llength $targets] == 0} return
+
 	if {[get_cmd_switch --skip-build]} return
 
 	regsub -all {\s\s+} $targets " " targets