diff --git a/build-arm-images.sh.in b/build-arm-images.sh.in
index 4f9adc4..0cf4fa1 100644
--- a/build-arm-images.sh.in
+++ b/build-arm-images.sh.in
@@ -3,7 +3,7 @@
 TARGET="$1"
 [ -n "$TARGET" ] && shift
 
-PLATFORMS="beaglebone cubieboard2 odroid-u2 rpi rpi2 usbarmory"
+: ${PLATFORMS:="beaglebone cubieboard2 odroid-u2 rpi rpi2 usbarmory"}
 DATE=$(date '+%Y%m%d')
 
 for f in ${PLATFORMS} x ${PLATFORMS} ; do