ART: suppress control characters in scripts without a tty

Makes LUCI output slightly more readable.

Test: manual
Change-Id: I00d51fe3815ded5e1478081e1b170c1eea200e6e
diff --git a/tools/setup-buildbot-device.sh b/tools/setup-buildbot-device.sh
index ee623e8..c2451d3 100755
--- a/tools/setup-buildbot-device.sh
+++ b/tools/setup-buildbot-device.sh
@@ -17,10 +17,13 @@
 # The work does by this script is (mostly) undone by tools/teardown-buildbot-device.sh.
 # Make sure to keep these files in sync.
 
-red='\033[0;31m'
-green='\033[0;32m'
-yellow='\033[0;33m'
-nc='\033[0m'
+if [ -t 1 ]; then
+  # Color sequences if terminal is a tty.
+  red='\033[0;31m'
+  green='\033[0;32m'
+  yellow='\033[0;33m'
+  nc='\033[0m'
+fi
 
 if [ "$1" = --verbose ]; then
   verbose=true